Spectra Remote Head Distance Limitations?

[k9nzf]

I have an application where I would like to use a 100W spectra with one head locally at the radio deck location and a second head about 250 feet from the radio deck. Now I know that the radio wasn't designed to do this, but that hasn't stopped many of us before! Between the radio location and the preferred head location, I have a 25 pair UNSHIELDED CAT3 24awg cable.I used 12 pairs in order up to pin 24 and then split the 13th pair and used one half of the pair for pin 25 on the DB25 connectors. So pins 1-2 share a pair, pins 3-4 share a pair and so on. Everything except for one little thing seems to work flawlessly. The mic audio is very noisy. The level seems to be OK(although, I haven't put it on a service monitor) but there is what sounds like white noise in the background. This noise is present on both heads. The noise is also present if the 250ft of CAT3 is connected WITHOUT the second head connected. It is present regardless of which DB25 on the radio I plug the CAT3 into. I have tried three different runs of CAT3 to eliminate a faulty cable. One other interesting characteristic is that if I hit a DTMF key on the mic, the transmitted carrier and DTMF audio are are as clear as a bell. But as soon as I use the PTT button to key the radio and make the mic element hot, the noise is there. The fact that the noise is there if the CAT3 itself is plugged into the radio leads me to believe that there is some sort of property about that cable that the radio doesn't like. I am thinking capacitance. Or is it possible that it is noise from the data lines that are supposed to be shielded? If so, why wouldn't it be picking it up when using DTMF as well? At first I was thinking that maybe I need to look at the pinout and see what pins need to be paired with respect to ground, but now I'm not so sure. Maybe I just need bigger gauge cable. I still have some experimenting to do, but I thought I'd see if anybody had any ideas or if anybody has tried this before.


Thanks for reading,

Jordan

[Jim202]

You need to take a look at the batlabs site and get the pin connections for the DB25 connector. The mic hi wire needs to be shielded. There are a couple of other wires that should be shielded.

Problem with the Spectra is that it uses a digital bus control for several functions in the radio. Like changing channels uses the digital bus to make it work and display the channel info on the display.

Most digital control buss lengths need to stay under like 100 feet. If you need to go longer, you might consider using a DC remote control or tone control. But now your limited to staying on a single frequency.

Jim

[desperado]

Motorola had a factory part number for a 150 foot control head cable.
They were used to connect a head in a ladder truck.
Beyond 150 I couldn't say.... and that is a factory cable, and not cat 5

[k9nzf]

Thanks for the responses. The data bus does not seem to be affected by the lack of shielding or length of the cable *yet* anyway. Nothing weird has happened regarding the display data or controls, but I haven't actually put it to use yet.

I think what I will do is set up a couple of additional test cables and use shielded cables for the Mic Audio and the data bus (just in case it is some sort of crosstalk) and see if anything changes.

As for the tone/DC remote idea. I have considered that, but I am trying to get as many channels as possible so unless I really want to spend some bucks, that is not going to work. And if I were going to spend those bucks, I would use something way newer than a Spectra. Spectras are cheap now a days and still good radios so I would like to see if I can make this happen.


Thanks again and I will update you when I make some more progress.


Jordan

[k9nzf]

Thanks guys,

I will do some testing with some other cables and see what I come up with.

Jordan

[k9nzf]

UPDATE:
I Still have to do some testing over time, but I have solved the problem by removing the Mic hi and Mic lo lines from the 25pr CAT3 cable and feeding them to the control head on a separate shielded CAT5 cable with the shield drain tied to the Mic lo line. I used a whole pair for each line instead of using a single twisted pair for both lines. I will test a single pair at some point though. So far it is working flawlessly at 250'.

Jordan