[MRCA] Retransmit Project Phase 4

[W2HX]

Ok folks, the next installment of the VHF/HF cross-band retransmit project.

Just to reiterate these are the phases of testing so far.
Phase 1 - get HF radio working on low power and sincgars VHF radio working on low power individually and use retransmission cable between them (completed successfully, or so I thought) 
Phase 2 - get sincgars radio installed in vehicular mount with high power RF amp, test radio, mount, amp and AS-3900 antenna (completed successfully) 
Phase 3 -configure both radios for high power (not interconnected) and perform tests to observe interference and other possible nastiness. (completed successfully)
Phase 4 - (current phase) interconnect both high power radios using the retransmit cable and perform full system testing. This would be using a PRC-68A HT as the VHF "client" radio and a BCC39B manpack as the HF "client radio." 
Phase 5 - (future phase) test gas generator and attempt to reduce ignition noise observed in the radios while operating 60 meters. Determine if gas generator is a viable power source. Also work to interconnect a 100AH 28V Li-Ion battery as a potential source of power for field operations.

While testing phase 4 I ran into a problem. I since realized that this problem existed back to phase 1, but I didn't test phase 1 carefully enough to realize it. The problem is as follows.  The speaker output of the PRC-138 follows the volume control as you would expect.  So the gain going into the MIC input of the RT-1439/PRC is adjustable.  And because of this, the audio sounds great! The audio from HF to VHF sounds beautiful on my "client" radio of the PRC-68A.  However, the volume of the speaker output of the RT-1439 - when using the RXMT connector - is fixed output and very high level. Therefore, the MIC input of the PRC-138 was being severly overdriven.  Unforuntately, due to inadequate testing in phase 1, I did not catch it earlier.

I took a few days to decide how to proceed. There were many options available to me. Some examples:
1. I could have used the DATA connector on the PRC-138 which has signals for FIXED RX and TX audio like the RXMT connector on the RT-1439. The problem here was this DATA connector has the right audio but does not have the Retransmit Keyline Ouput signal used to key the other radio. So I would have had to make a hybrid connection using some signals from DATA and the RXMT Keyline signal from the main audio u-229 jack. This seemed like a lot of work.
2.  On the RT-1439 I could have used one of the standard u-229 audio connectors where the speaker output follows the volume control. However, the audio connections did not carry the retransmit keyline output. I opened the radio and contemplated running a simple wire from the RXMT connector to the J3 or J4 (u-229 handset connector) and I would have everything I needed in one connector.  However to do this, I would have to disable the existing function of the "E" pin on one of these connectors. If I had chosen to proceed, I would have disabled J4 pin "E" which contained a signal used for the FILL function.  I had decided that I would never use FILL and so I would not be losing much.   To do this, there would be some very simple modifications to the RT-1439 and unfortunately because of the construction on the radio, I would likely not be able to restore this FILL function (short of buying a new front panel).  However, this approach would have been incredibly simple. Snip one pin on an internal connector, run one little wire from place A to place B and its done.

In the end, with the input of some smart friends, I decided to not modify the radio and go a third route. I created a little breakout board.  I took a cable that had 2 u-229 connectors (and all 6 conductors in use) and I cut it in half. I soldered each connector to a perfboard. This perfboard allowed me to interconnect the two radios as necessary - swapping RX and TX audio and PTT and RXMT keyline signals.  But most importantly, it allowed me to put a potentiometer between the RT-1439 audio out signal and the PRC-138 mic input.  You can see the pot at the top of the perfboard and several signals getting "crossed."   Here is a picture of what I built:
http://www.w2hx.com/x/RXMTProject/breakout-cable.jpg

Well, it worked! Mostly.  So I can sort of control the audio going into the MIC of the PRC-138. But the PRC-138, as many radios do, has very aggressive compression and audio processing that is always trying to maximize the TX signal.  So almost any adjustment of the pot produced little effect as long as it wasn't 100% (which is what it was before the pot was introduced).  So it definitely works, but here is an interesting issue. While a client VHF radio (my prc-68a) is keyed but between words, when there is no audio, the PRC-138 jacks up the gain trying to hear the VHF radio. This has the effect of boosting the normally very low "grunge" sound from the 150Hz squelch tone. This tone is normally barely audible, but when the PRC-138 jacks up the gain between words it becomes quite noticeable.  But as soon as the user speaks, the gain is reduced to support the voice and the grunge is no longer heard.  I will try to disable the compression menu option on the PRC-138 to see if this helps.

However, for now this will have to do, as I hope to deploy this system Friday at the Gilbert, PA.  In the future, I might try to add a high pass filter to this perfboard. Something maybe 300+ Hz but it would need to be very steep profile.  If anyone has a simple circuit that would greatly attenuate the 150 Hz signal, let me know. 

The weather here has prevented me from fully testing the high power version of this setup, but because phase 3 was successful and now, phase 1 has been revisited and it works, I have no doubt that this will work fine. If the weather cooperates tomorrow, I will confirm.

If you're interested, here is an interesting thing. The picture below shows a fully functional, low-power, VHF/HF, cross-band retransmit system that runs on batteries and can be deployed anywhere.  You may want a better HF antenna such as a dipole, but 20W on the right band could probably be very useful. Very simple, two radios and one interconnect.

http://www.w2hx.com/x/RXMTProject/low-power-xband-rxmt.jpg

Phase 5 of this project was to test the whole setup (high power) on battery power.  I have obtained a 28V, 100AH li-Ion battery and I am currently charging it. I have another 6 hours to go to get it to 95 AH. It will be ready for testing tomorrow as well. I have Anderson power poles on the battery (with an 80A fuse - battery is rated for 180A continuous) and should be easy to test.

Hopefully tomorrow I will have completed all the testing for this project. It has been fun, very educational and very time-consuming!  I will report back on how well the system works after this weekend when I hope to deploy it.

Thanks everyone for your comments, encouragement and advice. 

73 Eugene W2HX