View Full Version : Yaesu FT-7900R maximum faceplate separation?

03-08-2011, 01:31 PM
How far can I separate the Yaesu FT-7900R faceplate from the rest of the radio?

I wonder if I could run 100 feet of cat 6 23 AWG between the faceplate and the bulk of the radio? Could I run the external speaker wire 100 feet too?

Why would I want to do this?

I could put the radio much closer to the existing ground system instead of routing 100' of copper around the outside of the house. I could reduce my coax loss because I wouldn't have to traverse the house horizontally.

03-08-2011, 01:53 PM
Its OK with me if you do it.

03-08-2011, 01:57 PM
I think there is 9V on that line that feeds the regulator in the head unit. AFAIK nothing uses 9V but there is a linear regulator to supply the 5V, which if it's a typical 78L05 sorta thing it will remain in spec down to about 7V supply before the regulated side dips. Things might even run significantly below 5V, too.

If you're talking regular Cat-5 cable you're looking at probably 26AWG with a load of maybe a few watts, say 1A for nice numbers. You're looking at maybe 40 milliohms per foot, so 100' is 4 ohms, which is 4V at 1A, which puts the 9V at 5V, which means the linear will probably be well below 5V. I think that load is probably an over estimate, although with the backlight and everything, I dunno.

Signaling-wise, I think it's just a twisted pair serial protocol, so 100' might be OK depending on drive strength. You'd have to look at the schematic to tell what driver they are using. I'd bet it would work alright, but 100' is pretty long.

I can tell you this, the control cable for the FC-40 works fine at 50'. It's got 13.8V on it, not 9V, though. I'd suspect that the FC-40 is designed to operate at a wider swing since it's assumed that your radio would be operating probably at 12.2V minimum, 13.8V nominal. So it can deal with probably a cable drop on the order of 5V.

BTW, you don't need to do the wide copper strips for VHF FM, though it can't hurt. Grounding is about references and as long as everything is referenced to the same ground then you won't create a loop. The problem would be more difficult with an earth referenced antenna, which most likely (just guessing) your VHF is not. It's probably got it's own radials (particularly since it's mounted high, there is not much chance of it coupling to earth). Point is a 100 foot long ground conductor is fine as long as your radio, power supply and antenna grounds all connect together at the same spot only and then there's only one path back to earth.

Now the idea of a shorter coax is very valid. Interesting. This is probably the best reasoning for doing this, or at least some combination that does work, like 50' of control cable allowing a 50' reduction in coax. BTW, you ARE using RG-8 for this, right? RG-8X and RG-58 have ridiculous losses at >50MHz. :-)

03-08-2011, 02:50 PM
Thanks Bruce. :/

03-08-2011, 02:56 PM
Dave: The cat 6 I have is actually 23 AWG. I just checked. It is the pain in the butt heavy stuff that I hate building cables with. I'm going to whip up a 100footer and test it out with it coiled in the truck since I have the truck here with me today. The coax I have is Beldin 9913 selected due to my length requirement. The losses are bad, but not tragic.

03-08-2011, 03:00 PM
Dave: The cat 6 I have is actually 23 AWG. I just checked. It is the pain in the butt heavy stuff that I hate building cables with. I'm going to whip up a 100footer and test it out with it coiled in the truck since I have the truck here with me today.
That should help a lot, betcha it works. If you really want to test it, coil it up and put your cell phone right in the middle of the coil. If your radio doesn't go nuts, you're set.

03-08-2011, 04:44 PM
Testing complete:

I still don't know the MAXIMUM distance you can have the faceplate from the radio, but it is more than 100' if you use plain old CAT6, 23 AWG UTP cable with RJ-11 ends wired up in the modern USOC cable standard with the brown pair just left hanging there unterminated.

I placed the coil of 100' of wire on the seat, placed my mobile phone on top of the pile, and even moved the phone to the middle. I never saw the radio faceplate do anything weird. All of the buttons seemed to work fine. The brightness of the backlight looked the same too. I heard k0nl on the 144.670 repeater unable to raise the party he was looking for, so I keyed up and requested a signal report including voice modulation. Report was very clear audio. no weak audio.

There is a chance that I saw elevated noise levels with the 100' of wire coiled on the seat, but I'm not sure. It seems like I had to make a squelch adjustment, but then I wound up using the radio with the squelch back in it's usual position of about the middle.

The noise difference on my cable tester showed an average of .02 mV difference in USOC vs. 568B wiring standards on the same cable in case anyone was wondering. I chose USOC because it is the telecom standard for 6 wire RJ-11 connectors like those used on the Yaesu faceplate connection. I didn't want to confuse anyone if the cable got reused someplace. I suspect I could get lower noise levels if I used a modified 568B with the blue pair left out.

I guess this means Bruce is right. As expected.

By the way, I love saying USOC. "You sock! And your structured wiring 'round here socks too!"

03-08-2011, 04:49 PM
Good deal. Looks like 23 AWG is about 20 milliohms per foot, so 100' is about 2 ohms. That probably really did make a difference. Keyed up at 50W with the engine off would be the worst case and sounds like it worked fine.