Batboard

I'm planning on starting a project to make a securenet capable repeater for work but I'm looking for some pointers before I start.

I have gathered a few parts up so far. I have a 5 pin radus vhf, two Bendix/King LMH 30 watt mobiles, and a 50 watt A5 spectra that are all available for the project. I'm working on getting a duplexer and have the diagram from Will for the maxtrac cheapo astro repeater mod which I assume will work for securenet as well.

The repeater would be used for surveillance work and would not be used everyday. However, when it would be used it would be fairly heavy traffic. Especiall if someone goes on a tirade about the boss.

My main concern at this point is the duty cycle. I'm trying to figure out the best way to go aobut it so that it doesn't overheat. It would be used in a small town and coverage should be good at just 10 or 15 watts at most.

Would an external amplifier be my best route or should I try to use a Hi power spectra and just turn it way down? Any ideas would be appreciated.

The cheapo Astro repeater mod for the Maxtracs will not pass DES type transmissions.

You'd have to use two Spectras with the repeater controller that I posted for it to work.

OK, thanks for the info. I was going off memory here from work and thought that anything that came in one side went out the other. Too bad, as it would have been neat to be able to use P25 if available too.

I guess it's "double encoding" in a sense then that will be needed (Recovered DES re-encoded back to DES).


How does that actually sound when done that way?

It'll repeat what it hears, but DES is two wide for it to work. I suppose you could run P25 with encryption on, but I haven't tested that.

Using Spectra's to pass DES, by decoding and then recoding is a waste of time and will degrade the DES signal. A proper way is to get a DES box from a Syntor or Syntor-X. Run the signal from the RX discriminator to the DES box, tap off the reclocked data and insert into the TX radio. You can even make it proper code repeat by installing a hybrid and loading a key, so it will only transmit with the proper key. Another way is to use 2 MX-S secure portables. Do the same by pulling the reclocked data off the DES module in the RX radio and send it to the TX radio. It's a bit of work, otherwise your DES range will be severly limited. As far as p-25 I havn't worked on that yet. BTW none of the Maxtrac, Maratrac, or Radius line of radios will work with DES, believe me I tried.

That sounds do-able, but I have a few questions.

Could I use a remote spectra with an ASN box? I have both that are available for the project.

So, basically what I'm thinking is that I would setup the spectra with the external ASN box just as normal and use it as the receive radio. The CVSD data would be tapped somewhere within the ASN box and then routed out by way of a shielded cable to the flat audio input of the transmitting spectra.

If that's all correct then I guess the only issue is finding the tap point inside the ASN? If so, what should I be looking for? Is it tapped right off one of the hybrid pins?

I can see how audio quality might suffer from the decode-encode just using a couple of secure radios but, since the TX radio is doing DES encoding, I don't see how it is going to do anything with operating range (RF range). Care to explain?

DES requires strict tolerances, so with a double vocoded repeater, the signal gets garbled. Even though you can TX and RX, it'll only do so within close proximity to the repeater (depending on where the repeater is located.) For me, I have excellent coverage in an 8 mile radius in Secure mode. This is with the Spectra repeater that I made (about 15 Watts out and antenna about 30 feet above average terrain). The range increases to about 12 miles (6 miles from the repeater site vice 4 in Secure mode) when it is not in Secure mode. These measurements were all done via handheld radios. No mobiles were harmed during the documentation of these tests.

If I were to use a regular Secure repeater, then I'm certain the range would be the similar regardless if you were using DES or not.

From my understanding, DES is viable over shorter distances than a clear signal, period - the end. I don't think that a "real" securenet repeater is going to have longer DES range than two spectras back-to-back like you made - given equal RF characteristics for the two repeaters.

If the RX and TX are equal between the two, they're going to do the same RF wise and should do the same DES wise. The only downfall of using two spectras and doing decode-reencode is going to be audio quality.

Can you explain to me how it could be otherwise?

I think you're right. If it's operating properly it should perform like any other secure capable repeater.

What I would like to do is put it on top of the COOP tower on the North side of the down town area. That would put us 200-250 feet above everything else.

I just wonder how much of a pain it would be to use a Spectra with the built-in hybrid and DES board. You'd only need one with the hybrid and DES module.

Wouldn't you just have to pull the reclocked data and insert it into the TX radio? If so, does anyone have the location for that pin?

The secure repeater I made is offline now, but I think I am up to experiment again.

Seems like that would work for encrypted only. For clear/decrypted, you would have to somehow distinguish when the radio was receiving valid crypto and ONLY pass the reclocked data to the TX radio. Otherwise, the recovered audio going into the TX radio in parallell with the reclocked data is surely going to wipe out the data.

I'd love to have one that would pass crypto and clear, based on Spectras.

Heck, for that matter, it would be nice to have an even more "portable" version based on Sabers.

Even if you told the RSS to receive clear and secure comms? If I get some time on my hands, I'll try it out.

[edit] Just had a thought. What if you were using a controller that had two or more audio RX lines? I'm using an ICS controller that has two COR and two audio RX lines.

There's the control RX audio line and the repeater audio line. Anytime the control receiver line is active, the repeater line takes a back seat. Couldn't you have the reclocked DES line on one and regular audio on the other? Or does the reclocked DES audio line always stay active?

Just trying to think outside of the box.

Wouldn't the radio (spectra) take care of the switching automatically? At some point it has to toggle between either recovered CVSD or analog.

If I could find my spectra service manual I would look it up again but it seems like the square PLCC chips were what took care of the audio routing.

I tried downloading the service manual from MOL, but the downloaded versions don't have the schematics...

I would assume that the Spectra would take care of automatically switching between coded and clear as long as you had the right pin that you were pulling the audio from. Does your service manual include the hybrid pinouts? (wishful thinking).

Scott,


Can you contact me off-line at [email protected] I need some info
on setting up two spectras back to back for crossband operation.

Thanks in advnce. Joe