Saturday, 26 July 2014


It was a busy night on 475kHz WSPR - many stations in Europe active and more in RX only mode. Best DX for me was again with IW4DXW - we both "exchanged" spots over a 1927km path.

My best DX in the RX direction was to get 4 seprate spots from the USA - WA2XKA - run by John WA3ETD - in Pittsford, Vermont at a distance of 4950km.

Here is a list of each station either heard by me, or who heard me, overnight last night

 2014-07-26 01:52  WG2XKA  0.475722  -23  0  FN33lq  1  GM4SLV  IP90gg  4950  42 
 2014-07-25 20:42  EA3NE  0.475732  -24  0  JN11an  0.5  GM4SLV  IP90gg  2094  355 
 2014-07-25 22:20  GM4SLV  0.475728  -19  0  IP90gg  0.2  IW4DXW  JN64bw  1927  146 
 2014-07-25 21:40  IW4DXW  0.475605  -16  0  JN64bw  100  GM4SLV  IP90gg  1927  337 
 2014-07-25 21:28  GM4SLV  0.475729  -27  0  IP90gg  0.2  EA2HB  IN93ah  1886  181 
 2014-07-25 22:30  GM4SLV  0.475728  -28  0  IP90gg  0.2  SP5XSB  KO02ld  1642  113 
 2014-07-25 21:06  GM4SLV  0.475726  -19  0  IP90gg  0.2  DH5RAE  JN68qv  1578  137 
 2014-07-25 21:40  DH5RAE  0.475751  -23  0  JN68qv  0.2  GM4SLV  IP90gg  1578  329 
 2014-07-25 23:44  DJ0ABR  0.475760  -29  0  JN68nt  0.2  GM4SLV  IP90gg  1576  329 
 2014-07-25 22:42  GM4SLV  0.475727  -25  0  IP90gg  0.2  DJ0ABR  JN68nt  1576  137 
 2014-07-25 20:44  F1AFJ  0.475703  -13  0  JN06ht  1  GM4SLV  IP90gg  1503  356 
 2014-07-25 20:38  GM4SLV  0.475726  -26  0  IP90gg  0.2  F1AFJ  JN06ht  1503  174 
 2014-07-25 20:56  GM4SLV  0.475725  -23  1  IP90gg  0.2  OH2SWL  KP20uj  1485  78 
 2014-07-25 22:00  GM4SLV  0.475737  -24  0  IP90gg  0.2  DF6SM  JN58ct  1475  145 
 2014-07-25 21:16  GM4SLV  0.475726  -27  0  IP90gg  0.2  F59706  JN07th  1455  171 
 2014-07-25 20:58  DK6NI  0.475672  -23  0  JN59ln  0.1  GM4SLV  IP90gg  1426  331 
 2014-07-25 21:00  DK2DB  0.475641  -12  0  JN48fw  0.5  GM4SLV  IP90gg  1409  337 
 2014-07-25 21:06  GM4SLV  0.475727  -16  0  IP90gg  0.2  DK2DB  JN48fw  1409  149 
 2014-07-25 21:06  GM4SLV  0.475725  -20  0  IP90gg  0.2  DF8UO  JN48ex  1403  149 
 2014-07-25 22:56  F6HCC  0.475678  -24  0  IN87hs  0.1  GM4SLV  IP90gg  1395  4 
 2014-07-25 20:56  GM4SLV  0.475723  -25  0  IP90gg  0.2  SP2OVY  JO94hg  1372  110 
 2014-07-25 21:16  GM4SLV  0.475729  -22  0  IP90gg  0.2  DK7FC  JN49ik  1366  147 
 2014-07-25 20:42  F5WK  0.475652  -18  0  JN18hp  0.2  GM4SLV  IP90gg  1319  350 
 2014-07-25 20:38  GM4SLV  0.475725  -26  0  IP90gg  0.2  F5WK  JN18hp  1319  167 
 2014-07-25 22:00  GM4SLV  0.475727  -24  0  IP90gg  0.2  F6CNI  JN19qb  1284  164 
 2014-07-25 21:46  DH0PAZ  0.475662  -23  0  JO30nm  0.05  GM4SLV  IP90gg  1210  337 
 2014-07-25 20:38  GM4SLV  0.475724  -29  0  IP90gg  0.2  DH0PAZ  JO30nm  1210  150 
 2014-07-25 22:20  GM4SLV  0.475727  -28  0  IP90gg  0.2  SM2DJK  KP03au  1182  61 
 2014-07-25 21:36  GM4SLV  0.475726  -29  0  IP90gg  0.2  G3XKR  IO70ux  1048  191 
 2014-07-25 22:08  GM4SLV  0.475728  -19  0  IP90gg  0.2  G8JNJ/A  IO90hx  1033  180 
 2014-07-25 22:08  GM4SLV  0.475728  -27  0  IP90gg  0.2  PI4THT  JO32kf  1030  147 
 2014-07-25 20:38  GM4SLV  0.475724  -24  0  IP90gg  0.2  PA3ABK/2  JO21it  1014  155 
 2014-07-25 20:40  PA3ABK/2  0.475740  -1  0  JO21it  0.5  GM4SLV  IP90gg  1014  340 
 2014-07-25 21:06  GM4SLV  0.475727  -19  0  IP90gg  0.2  DG3LV  JO53gv  1011  129 
 2014-07-25 20:48  DG3LV  0.475786  -27  0  JO53gv  20  GM4SLV  IP90gg  1011  319 
 2014-07-25 21:36  GM4SLV  0.475726  -22  0  IP90gg  0.2  G6AVK  JO01ho  972  171 
 2014-07-25 21:32  G6AVK  0.475762  -27  0  JO01ho  1  GM4SLV  IP90gg  972  353 
 2014-07-25 22:08  GM4SLV  0.475728  -17  0  IP90gg  0.2  M0PPP  IO93gm  751  180 

Here's the 4 spots of WG2XKA :

 2014-07-26 02:14  WG2XKA  0.475722  -27  0  FN33lq  1  GM4SLV  IP90gg  4950  42 
 2014-07-26 02:04  WG2XKA  0.475721  -25  0  FN33lq  1  GM4SLV  IP90gg  4950  42 
 2014-07-26 01:58  WG2XKA  0.475721  -23  0  FN33lq  1  GM4SLV  IP90gg  4950  42 
 2014-07-26 01:52  WG2XKA  0.475722  -23  0  FN33lq  1  GM4SLV  IP90gg  4950  42

A map view of the transatlantic crossing:

Friday, 25 July 2014

Return to MF

I last used MF under the old UK Special Research Permit, on 501 - 504kHz, in November 2011 after experimenting on the band since July 2007. Since that time the WRC has produced a new band worldwide at 472 - 479kHz. Not all administrations have released this new band but there are certainly more countries with access to MF now than when I was last using it.
I had a chance CW QSO on 5MHz with Chris G3XIZ, one of the MF stalwarts, earlier in the week and we discussed the idea that I might come back to MF. I got thinking about the good and interesting times I had between 2007 and 2011, with many CW contacts with Finbar GI4DPE and later EI0CF, and the interesting one-way tests between Shetland and Finland with Paul-Henrik OH1LSQ. He would now have access to MF and could now transmit as well as receive, so now we'd be able to have in-band two-way QSOs instead of cross-band or one-way (report by email) tests.
I dragged my MF transmitter from under the stairs and set about testing it and retuning the PA stage from 501 to 472kHz. It was soon producing 100W into test load. 

Next I retrieved the MF ATU (basically a tapped loading coil and an impedance matching transformer) and put it at the feedpoint of my current HF inverted-L. It didn't take long to tune & match the antenna - although it was much shorter, electrically, than the previous MF antenna - needing an extra 200uH of inductance to get it to resonance. After a day of testing I replaced the antenna with a new inverted-L, 11m high with a 25m long top-loading section. This initially resonated at 360kHz with the same tuner settings, a big increase in capacitance due to the extra top-loading. Removing the additional inductor and retuning got me a good match - the matching transformer ratio showed an overall resistance (mainly due to ground losses) of around 35 ohms. This is the same figure as when last used on MF, so the earthing system seems to be intact! 12.5W from the transmitter produced an approximate antenna RF current of 0.6A, which also implies a resistance of 35 ohms.
An approximate calculation, using Rik's excellent LF antenna Guide as reference, shows the expected Radiation Resistance to be 0.3 ohms
For 100W TX power I'd expect 1.7A of RF, and 867mW EMRP. This is an ERP of 1.5W and an EIRP of 2.6W. The UK limit is 5W EIRP. I have some way to go to reach the EIRP limit!
A couple of nights using WSPR, to give it a test... here's last nights "best DX" : 

 2014-07-24 21:22  GM4SLV  0.475724  -28  0  IP90gg  0.2  IW4DXW  JN64bw  1927  146 
 2014-07-24 19:10  IW4DXW  0.475605  -28  0  JN64bw  100  GM4SLV  IP90gg  1927  337 
 2014-07-25 02:58  GM4SLV  0.475731  -28  0  IP90gg  0.2  EA2HB  IN93ah  1886  181 
 2014-07-24 21:22  GM4SLV  0.475722  -33  0  IP90gg  0.2  DF8UO/A  JN37vo  1530  153 
 2014-07-24 20:22  F1AFJ  0.475704  -21  0  JN06ht  1  GM4SLV  IP90gg  1503  356 
 2014-07-24 20:32  DK6NI  0.475682  -28  0  JN59ln  0.1  GM4SLV  IP90gg  1426  331 
 2014-07-24 21:24  DK2DB  0.475641  -6  0  JN48fw  0.5  GM4SLV  IP90gg  1409  337 
 2014-07-24 21:22  GM4SLV  0.475724  -24  0  IP90gg  0.2  DK2DB  JN48fw  1409  149 
 2014-07-24 20:46  GM4SLV  0.475723  -28  0  IP90gg  0.2  DF8UO  JN48ex  1403  149 
 2014-07-24 20:38  GM4SLV  0.475726  -30  0  IP90gg  0.2  DK7FC  JN49ik  1366  147 
 2014-07-24 19:58  F5WK  0.475652  -26  0  JN18hp  0.2  GM4SLV  IP90gg  1319  350 
 2014-07-24 20:46  GM4SLV  0.475725  -22  0  IP90gg  0.2  F5WK  JN18hp  1319  167 
 2014-07-24 21:44  GM4SLV  0.475723  -18  0  IP90gg  0.2  G3XKR  IO70ux  1048  191 
 2014-07-24 22:16  GM4SLV  0.475724  -18  0  IP90gg  0.2  G8JNJ/A  IO90hx  1033  180 
 2014-07-24 21:10  GM4SLV  0.475723  -27  0  IP90gg  0.2  PI4THT  JO32kf  1030  147 
 2014-07-24 18:30  GM4SLV  0.475726  -31  0  IP90gg  0.2  PA3ABK/2  JO21it  1014  155 
 2014-07-24 18:40  PA3ABK/2  0.475740  -3  0  JO21it  0.5  GM4SLV  IP90gg  1014  340 
 2014-07-24 20:50  DG3LV  0.475786  -25  0  JO53gv  20  GM4SLV  IP90gg  1011  319 
 2014-07-24 20:58  GM4SLV  0.475725  -21  0  IP90gg  0.2  DG3LV  JO53gv  1011  129 
 2014-07-24 21:10  GM4SLV  0.475723  -26  0  IP90gg  0.2  G3WCB  IO91rm  975  176 
 2014-07-24 22:16  GM4SLV  0.475724  -15  0  IP90gg  0.2  M0PPP  IO93gm  751  180 
 2014-07-24 22:02  M0PPP  0.475698  -16  0  IO93gm  5  GM4SLV  IP90gg  751  0

I've set up a "grabber" using my Perseus SDR which can cover the full 7kHz of the new band. To improve the resolution of the images I've split the band into two sections. The grabber is here:

Next I plan some antenna improvements. I'm limited to the height I can raise the vertical section of the antenna, but I can probably add more top loading, and hopefully improve the ground system to reduce the loss resistance. Reducing ground losses leads to an increase in RF current flow in the antenna, and as radiated power is proportional to the square of the antenna current, every extra ohm shaved from the ground loss increases the ERP. Dropping to a ground loss of 30 ohms would improve the E(I)RP by around 0.6dB. Adding more top loading would improve the current flow, increasing the radiation resistance and also reduce the system losses by reducing the losses in the ATU (as the antenna would need less inductance to bring it to resonance) and this would also add to the ERP. I'm wary about adding too much wire, as I know from previous experience what a Shetland winter storm can do to wires hanging in the air...

Monday, 14 July 2014

Night of Nights 2014

I've followed the story of the USA's "Maritime Radio Historical Society" and their restoration of the former coast station KPH at Point Reyes/Bolinas in California over the last few years. For background reading they have a nice website : and various You Tube videos:

Each year, on the anniversary of the initial closure of KPH they bring the station back to the air for "Night of Nights", and have pulled in other former Morse Coastal stations who still have facilities, such as WLO in Alabama. This year I wanted to see if I could hear any of the action.

KPH and most of the other stations are on the West Coast, and mid-summer, with static crashes and little darkness, means hearing them on MF or the low HF bands is unlikely. VOACAP showed a chance on bands between 8 and 17MHz. WLO in Alabama was likely to be a little easier to hear, although again, 500kHz was never likely to be successful.

The event was due to begin at "one minute past 5 pm" PST - one minute after the anniversary of the shutdown on July 12th 1999.

This is 0001 UTC which is 01:01 BST!

The MRHS published the planned frequencies of the participants in their newsletter

I programmed the frequencies into my HF radio and settled down for a late night looking for weak CW.

At 2045 UTC I heard "NMC de KKUI" on 16736kHz and then shortly afterwards "WLO do KKUI" also on 16736kHz. KKUI is the callsign of "American Victory", a museum ship anchored in Tampa, Florida.

Later I began to hear WLO sending it's "wheel", the repeating channel announcement listing their listening frequencies. I could hear them on their 8MHz, 12MHz and 16MHz channels, although shortly afterwards the 12MHz channel fell silent.

Here's a recording from 16968.5kHz

At 0001 UTC I began to hear, very weakly - affected by static crashes and all manner of polar fluttering etc. a barely detectable Morse signal from KPH on 17016.8  and in parallel from KFS on 12695.5  and 17026.0

The best recording I could manage is here

Not good, but with a bit of imagination you can hear "VVV" and "KPH" and a few other things drifting out of the noise.

At 0030 UTC I again heard "American Victory" KKUI calling KPH, and heard the KKUI side of a contact with the former coast station. Quickly checking the frequencies used by KPH showed nothing I could read, so I've only got the KKUI side of the QSO:

After this QSO with KPH I heard KKUI calling the partner station of KFS, and again captured the KKUI side of the exchange:

There were also amateur stations active at KPH and WLO - K6KPH and W4WLO respectively. I heaerd WLO's wheel giving the frequencies, but heard nothing of W4WLO - but there was a big amateur radio contest in full flow (the IARU HF Championship). I did hear a station, K4JJW, calling K6KPH briefly on the amateur 17m band (18097.5kHz):

I lasted until 2am local time before calling it a night. I'm sure more traffic would have been audible later, especially on the lower channels, 12MHz and 8MHz.

I checked the channels at around 0700 UTC and heard weakly the final closing message, not really readable, but the odd snatch was clear enough to determine what the signal was, on one of the 8MHz channels used by KPH/KSM/KFS

Looking at WLO's channels and I found their closing statement, ending with the first message sent by Samuel FB Morse: "What hath God wrought?"

I enjoyed knowing that these old transmitters were back on the air, imagining the lights dimming on each dot and dash when keying multiple high power transmitters in parallel. It's a pity that none of the European power houses of the past have been conserved and could join in this celebration and remembrance of the history of Marine Radio using Morse Code. At least we still have the MRHS to keep CW alive.

Wednesday, 9 July 2014

Update on VHF/HF Cross-gate

The Voice Operated Squelch used in the first test version was okay for detecting human speech, but suffered from false triggering on static crashes from thunderstorms. Further research showed that the kit I got via eBay is a copy of the Naval VOS-4 design :

This is also the squelch circuit used by Steve G4HPE in his experimental HF-Internet Gateway:

I decided to try the "original" syllabic squelch circuit, as published in 73 Magazine in 1982 and is a copy of the Motorola "Constant SINAD Squelch" which was originally part of their MICOM range of HF SSB radios:

This proved to be much more successful, and since it's adequately documented it is possible to make changes to component values to alter the behaviour to suit different purposes. The Jet Propulsion Lab modified this circuit to use on a link feeding NASA-Select downlink audio into a VHF Repeater:

I'm planning to replace the V/UHF link transceiver - currently an FT817 which gets warm even when on Low Power during a long "HF to UHF" transmission - installing an FT8900R which is capable of higher power, and also has a cooling fan. The external "data" interface is identical to the FT817 and therefore can easily be swapped in without re-making cables.

Since this is effectively "Remote Operation" under Clause 10 of the UK Amateur Licence, the 70cm port can run at any power (up to the maximum allowed by the license) as the "communication link" to operate the main station

10(1) The Licensee may conduct Unattended Operation of Radio Equipment provided thatany such operation is consistent with the terms of this Licence. Additional restrictions whichapply to the Unattended Operation of Beacons are specified in Schedule 2 to this Licence.  
10(2) Subject to Clause 10(3), the Licensee may also conduct Remote Control Operation ofRadio Equipment (including, for the avoidance of doubt, Beacons) provided that any suchoperation is consistent with the terms of this Licence.  
10(3) This Clause 10 does not permit the Licensee to install Radio Equipment capable ofRemote Control Operation for general unsupervised use by other Amateurs.  
10(4) Any communication links used to control the Radio Equipment or to carry Messages to or from the Radio Equipment in accordance with Clause 10(2) must be adequately secure so as to ensure compliance with Clause 3 of this Licence. Any security measures must be consistent with Clause 11(2) of this Licence. 
10(5) The use of any such communication links referred to in Clause 10(4) must be failsafesuch that any failure will not result in unintended transmissions or any transmissions of atype not permitted by this Licence. 
10(6) If this Licence is a Foundation Licence or an Intermediate Licence, and the Licenseewishes to establish communication links to operate the Radio Equipment in accordance withClause 10(4), then the Licensee may only do so using wireless communication links and theLicensee may only use the amateur band allocations detailed in Schedule 1 to operatethose links. Any such communications links shall be subject to a maximum power level of500 mW pep e.r.p. 
10(7) Only where this Licence is a Full Licence, Full (Reciprocal) Licence, Full (TemporaryReciprocal) Licence or a Full (Club) Licence, the Licensee may make use of anycommunications links (including, for the avoidance of doubt, the amateur band allocationsdetailed in Schedule 1) to establish the wireless communication links referred to in Clause10(4).

Clause 10(4) is taken care of by

  1. Using DCS on the 70cm port, with "code" which is is regularly changed.
  2. Using DTMF to control the talk-through, which requires a 4-digit "PIN" number before the instruction to enable/disable the HF-UHF and the UHF-HF talkthrough. The ability to enable each direction separately allows for "HF Monitoring" without enabling "HF Transmit" and only enabling the UHF-HF direction when two-way comms is required.
The link can't be "encrypted" as that's forbidden by the general licence conditions, but I think the steps of "DCS" to access the UHF port and "DTMF" to enable/disable talkthrough meet the terms of clause 10(4):

10(4) Any communication links used to control the Radio Equipment or to carry Messages
to or from the Radio Equipment in accordance with Clause 10(2) must be adequately secure
so as to ensure compliance with Clause 3 of this Licence.

I've used the remote link to have QSOs with GM4WMM by operating the home station on 5.298MHz while I was located approx 26km from home, and the system worked well.

Thursday, 26 June 2014

First Place in CQWW

I dabbled in the CQWW SSB Contest last year and submitted a log. I've just checked the CQWW results and it appears I came First in my country for the Single Op, All Band, Low Power category!

Since Shetland is a separate country for CQWW, and there were only 3 active stations in the contest

Saturday, 21 June 2014

HF - VHF Cross-gate

I've been playing with the idea of being able to cross-band repeat between HF and VHF, after reading about the Barrett 2062 x-gate on their website:

Initial tests last year, just using the simple RF-level squelch from the HF transceiver to trigger transmit PTT on the VHF port proved rather poor, as fairly readable HF SSB signals were not causing the squelch to open. What was needed was a syllabic squelch - as found on professional HF SSB radios - for the HF port. Using the VHF transceiver's normal FM squelch is perfectly okay, of course.

I found various designs for "Voice Activated Squelch" circuits and then stumbled on a kit on eBay:

I bought a couple and built one to play with, in series with the audio from the HF receiver. It certainly worked much better on weak SSB than an AGC/RF level squelch.

The Barrett device also used DTMF (or SELCAL) to activate the talkthrough, so I thought I'd try and emulate that, and found another kit that would fit the bill:

I built the kit, but never tested it. I consigned both the VOS board and the DTMF board to the "in progress" pile, and forgot about them for 12 months!

Today I thought I'd resume the project......

After modifying the VOS board to trigger a relay, which could be used as a PTT source for the VHF radio, I wired it all up on the bench, and it works, I even get the revertive CW signals from the DTMF board sent back over the VHF port when the commands are issued to turn the relay on & off.

Adding a few switches to manually inhibit talkthrough, or to override the DTMF driven active/inhibit relays and it's all boxed up and working. I need to add some LEDs, to show what's happening inside. The one main difference between this box and the Barrett 2062 is that the Barrett device allows remote channel changing of the HF transceiver via the VHF port - something that's not immediately possible in this simple version. I guess a PIC and some programming and it might be possible to send CiV signals tothe IC7200 on receipt of specific DTMF sequences, but that's starting to get more complicated!

Using the IC7200's rear-panel accessory socket also has the bonus that the Speech Processor is available, unlike the old IC718 (where the processor only worked on the front panel Mic socket).

Now I need to try it out on the air, 5MHz would seem the ideal place to test these things - so I need to set up a sked with a willing volunteer...


After much local testing, and quite a bit of listening, I heard MM1MAJ/P on 5403.5kHz operating from a SOTA summit near Loch Leven. Weak, barely moving the S-Meter on 5MHz but triggering the syllabic squelch in the x-gate reliably so I thought I'd try a call to him. He came back instantly and gave me a 5/7 report. The only minor issue is the few seconds the HF squelch takes to drop at the end of an incoming transmission, but that's no real problem. I'm quite pleased it worked on the first real on-air QSO.

I've thought a bit about HF channel changing, and the only simple solution seems to be to use one of the DTMF relays (there can be up to 6 relays) to trigger an up/down channel change via the Mic socket (pin 3). Unfortunately the "pulse" option of the DTMF board toggles a relay for 1/2 a second, which is too long for a single channel step. I will think about a pulse shortening circuit (differentiating the 1/2 second pulse and using that to click a relay for a brief instant.....?).

Or just leave the HF transceiver on one frequency.

I will try to monitor 5298.5kHz as this channel tends to be lightly used, and the regular occupants tend to mess about with odd setups too, so perhaps it's a good place to try some tests.

I've found it necessary to leave the DSP Noise-blanker turned on to remove the occasional (weak) clicking from a distant electric fence, which would occasionally trigger the syllabic squelch. I also find that night-time lightning static crashes can also be a nuisance, as they tend to occur at roughly speech pulse rate, I guess. Using the DSP Noise Reduction and (for good measure) leaving the Auto Notch Filter engaged might just be enough to keep things usable.

A 5-minute TimeOutTimer on the V/UHF transceiver (an FT817) means it's possible to recover control should something (a long...long...over for example) hold the x-gate open in the RXHF->TXVHF direction. Once the V/UHF rig "times out" it's possible to send the DTMF "OFF" sequence and kill the x-gate until the long QSO on HF is over, or the HF channel can be changed.

Tuesday, 17 June 2014

New box for GB3LU

Our old Icom repeater unit at GB3LU is starting to show its age - already the internal PSU has died and it was running on an external 12V PSU, and recently it's being turning itself off, for no obvious reason. Time for an upgrade....

I had a Simoco PRF1050 in my "rainy day" cupboard under the stairs that would do the job nicely, as long as it would interface with the existing Logic (a G8CUL designed controller I built some years ago). Checking it out on the bench showed that RX & TX Audio, Squelch (COR) and PTT were easily accessible via the 37-way facilities plug:

1Tx600 Ohm-14dbm nominal for 60% dev at 1Khz (Range -37dbm to 0dbm)
Transformer coupled to pin 20 or AF high if unbalanced
3Rx600 Ohm-14dbm nominal for 60% dev at 1Khz (Range -37dbm to 0dbm) Transformer coupled to pin 22 or AF high if unbalanced
4No connection made
5Common Station AlarmOpen collector low, when active
6Not assigned reserved for future use
7Not assigned reserved for future use
8CTCSSUnprotected input to TX modulator. Level and input impedance to be advised shortly
9+13.6v unswitched1 amp available to power external devices
10TX KeyPull down. Typical current 1.5mA. Open circuit voltage typically 5V
11TX Lock AlarmOpen collector low when activated
13RX Lock AlarmOpen collector low when activated
14CTCSS Decoder DefeatPull down to activate. When activated allows squelch to open with any signal
15RSSIRange as follows but can be negotiated if difficult to implement Graph attached
16Channel Line 6Binary C6, Normally high, pull low
17Channel Line 4Binary C4, Normally high, pull low
18Channel Line 2Binary C2, Normally high, pull low
19Channel Line 0Binary C0, Normally high, pull low
20TX600 Ohm-14dbm nominal for 60% dev at 1Khz (Range -37dbm to 0dbm)
Transformer coupled to pin 1 or AF return/ground if unbalanced
21No connection made
22RX600 Ohm-14dbm nominal for 60% dev at 1Khz (Range -37dbm to 0dbm)
Transformer coupled to pin 3 or AF return/ground if unbalanced
23External LS HighExternal loudspeaker typically 16 Ohms
24Squelch StatusOpen collector, low when squelch open
25Squelch Relay NC1 amp DC maximum at 48V DC. Floating
26Squelch Relay NO1 amp DC maximum at 48V DC. Floating
27Squelch Relay CommonFloating
28DC Input under voltage alarmThe PRF10 can charge external 12V Sealed Lead Acid battery. The battery can be damaged is subject to a deep discharge. The PRF10 automatically disconnects at a predetermined voltage. An alarm is signalled at 100mV prior to disconnect. Open collector low, when activated
29Talk Through EnablePull down to activate
30Squelch DefeatPull down to activate
31No connection made
32No connection made
33RF Output AlarmOpen collector low, when RF fails to meet power
34Unfiltered and unmuted RX AFEffectively discriminator output. Level and impedance to be defined shortly
35Channel Line 50Binary C5, normally high, pull low
36Channel Line 3Binary C3, normally high, pull low
37Channel Line 1Binary C1, normally high, pull low

The plan to interface with the existing GB3LU Logic :

TX Audio : PRF10 pin 1 - Logic pin 12
RX Audio : PRF10 pin 3 - Logic pin 10

Also Earth PRF10 pin 20, 22 (to PRF10 pin 12) for unbalanced audio

SQL - low on Squelch open :
Earth PRF10 pin 27 (SQL relay common)
SQL signal : PRF10 pin 26 - Logic pin 4

PRF10 pin 10 - Logic pin 7 (logic pulls this low to transmit)

Power for the Logic unit from PRF10 pin 9 - Logic pin 1
Ground : PRF10 pin 12 - Logic pins 14-25

This worked fine except for 2 problems....

1) The TX line-input greatly attenuated the CTCSS tones sent from the logic unit and it was not possible to achieve a sensible deviation for the 77Hz CTCSS on transmit.

2) The RX line-output also had a reduced amplitude of the RX CTCSS tone, which is used in the Logic unit to enable the talkthrough (we don't use old fashioned 1750Hz access, but require "continuous" CTCSS)

Problem 1 was solved by finding that there is a direct feed in from pin 8 for injecting external CTCSS tones. Initially this proved not to work as expected until I discovered there's a handbag link on the main PCB that connects the input from pin 8 through to the analogue circuits of the PRF10. Installing the missing link got the external CTCSS input working. I modified the Logic unit to feed a separate TX CTCSS signal from the CTCSS encoder chip into the PRF10 / pin 8. A pot on this feed allowed the deviation to be adjusted to around 500Hz. The driving factor with wanting to use the Logic unit's own CTCSS signal, and not simply configuring the PRF10 to transmit a 77Hz CTCSS tone, is that we carefully adjusted the timing of the Logic's CTCSS to aid the use of personal "cross-band repeaters" - and users who avail themselves of this were keen to maintain the function. This requires that the outgoing CTCSS is muted shortly after the "pip/K" end of over signal, allowing x-band repeaters to drop back to "idle". If the CTCSS was enabled continuously during the time the box is "open" then the only way to operate a x-band repeater is to let the box close down at the end of each over, and then re-access it once your x-bander has dropped out. Keeping the custom-timed CTCSS is therefore an important requirement. Also, as a pleasant side effect, the CTCSS is also muted during the regular repeater idents, so anyone choosing to monitor with "Tone Squelch" will only hear actual traffic.

GB3LU Logic Unit

Re-programming the controller

PRF10 under test

Problem 2 was investigated further....

I found that a normally modulated input signal (with > 300Hz deviation of the CTCSS) would trigger the Logic's CTCSS decoder, even though the 'scope showed the amplitude of the CTCSS tone at the input to the Logic to be only around 14mV rms, and a long way below the amplitude of the main audio signal. A problem still existed though - if an over-deviated signal was present I found that the CTCSS decoder dropped out on speech peaks, and then the talk-gate would close briefly- leading to breaks in through audio on speech peaks - but only with either very noisy or over deviated input signals.

There is a feed of un-processed "discriminator" audio from the PRF10 at pin 34 and this had ample CTCSS level, but also the full audio signal too. I built a 2 pole active LPF with a 300Hz cut-off and put it in series with the "raw" RX audio, and then paralleled this (via a 1k resistor) with the normal RX Line input to the Logic. This improved the detection of the CTCSS tone, it would open with a CTCSS deviation of around 200Hz, but simply tee-ing the two audio feeds (LPF feed of CTCSS and the de-emphasised main RX audio) wasn't the right solution. I would need to modify the Logic unit to provide a separate "CTCSS Input" directly to the CTCSS chip - fed from the LPF'd raw audio.

I decided to revert to the initial condition- one audio feed - and to re-programme the Logic back to "classical 2m repeater behaviour" in that CTCSS is only needed to initially open the repeater, and not to maintain talk-through. This seemed to be a decent compromise, and I also restored the 1750Hz tone-burst access too. The repeater was only configured for continuous CTCSS to overcome a problem with local interference on the repeater's input frequency, generated by a site-sharer at the repeater site, and it appears that the device responsible for this interference is no longer in use.

Bench measurements show that the RX sensitivity is good:

Squelch opens at -113dBm / 20dB SINAD with the PRF10 set to its minimum Squelch threshold (parameter 105.xx = 1). The squelch threshold at this setting  is nominally "9dB" but this isn't actually the case in reality. I experimented with varying PRF10 parameter 921 (factory set squelch calibration) and found:

As found : 077
Threshold : -113dBm / 20dB SINAD

Adjusted to : 020
Threshold : Squelch permanently Open
-117dBm  / 15dB SINAD

Adjusted to: 028
Threshold : -115.5dBm / 18dBm SINAD

I reset the parameter to the default value (077) for the time being. Squelch threshold can be revisited once the box has had a bit of on-air use, and after the antenna/feeder is replaced.

For reference the higher squelch settings:
Parameter 105.xx:
2 (nominally 12dB SINAD) : Threshold = -111dBm / 23dB SINAD
3 (nominally 15dB SINAD) : Threshold = -109dBm / 24dB SINAD

The various timings for the Logic are currently:

"Pip" time (from squelch close to sending the "pip/K" ) = 1 second
CTCSS off time (from squelch close to cutting outgoing CTCSS ) = 2 seconds
Off Time (from squelch closing to closing repeater if no further input) = 9 seconds
Toneburst length (minimum 1750Hz toneburst to be recognized) = 500mS
Latch Time (length of carrier required after toneburst to cause the repeater to open fully) = 1 second
Squelch Delay (minimum length of carrier required to cause an "ack") = 1 second
Timeout Time (overs longer than this will cause the repeater to "timeout" and then to closedown) = 10 minutes
Timeout End Time (period of "pips" marking the timeout before closedown) = 10 seconds

Beacon Time : 15 minutes
Send Locator with beacon : every 4 (i.e. once per hour)

To access : at least 500mS of 1750Hz toneburst or a valid 77Hz CTCSS and one second of carrier/audio
The repeater will key up briefly on input signals for shorter than this duration.

To prevent 10 minute Time-Out - ensure you wait to hear the "pip/K" at the end of the over before you start to transmit. This indicates the 10 minute timer has been reset. If a station transmits for > 10 minutes the repeater will cut the through audio and send a series of 10 one-seconds pips before closing down. Once the input signal has gone the repeater will key up and send "OK" in morse, and will then be available as normal.

A stronger signal can re-set a timeout that's in progress by sending a valid 1750Hz toneburst, or 77Hz CTCSS, as long as they can overcome the signal that's currently transmitting.

There is also a remote DTMF Closedown and Startup command - to be circulated to the Repeater Closedown Operators - which will cause the repeater TX to be inhibited or restored, by sending specific DTMF codes. This assumes you are able to overcome any signal on the input frequency that might be causing problems, but at least it will allow a remote shutdown, even if you have to travel nearer to site. If it means not actually having to get up the hill, and into the hut in a Shetland winter storm, that's a good thing!

GB3LU is back on the air as of June 13th 2014, using the PRF10 - notionally on "soak test".

GB3LU in the hut & on the air

Coverage is reduced at present due to antenna/feeder problems - and these will need to be addressed before the end of the summer.