Repeater or dual antenna?

[WRUE538]

I'm an old Navy METCAL tech, YouTube expert with zero GMRS knowledge. I've researched Dr. Fong antennas,  Fumei 50w duplexer, notch duplexer tuning, using NANO vnaH4, etc. Building a 50w repeater using 2 Wouxun KG1000's. I can locate 2 vertical antennas 20' apart on a conex container. My question is why would I rather even want a duplexer with no loss other than good cables and gain from antennas?

Thanks from Pryor, Colorado 

[gortex2]

It all depends on your expectation on how well the system will work. A duplexer helps keep desense to a minimum. The antenna is the most important part of the repeater. Spending quality funds on a good antenna and line twice is normally more money than a duplexer and one good antenna. Also if your going at a commercial tower you pay per antenna and line on the tower. I can say you can get a repeater that is designed as a repeater cheaper than you will get 2 KG1000's ...food for though

[WRUE538]

7 hours ago, gortex2 said:

It all depends on your expectation on how well the system will work. A duplexer helps keep desense to a minimum. The antenna is the most important part of the repeater. Spending quality funds on a good antenna and line twice is normally more money than a duplexer and one good antenna. Also if your going at a commercial tower you pay per antenna and line on the tower. I can say you can get a repeater that is designed as a repeater cheaper than you will get 2 KG1000's ...food for though

No commercial rent problem, already have the high ground and facility. Another 20' LM400 w/ connectors (will already need one) is $65, $49 for a Fong kit. Also less 2 jumpers to duplexer should be cheaper? I'll certainly look into the repeater being cheaper. I know there's used Motorola's in racks around but not sure about 70cm bandwidth. Thank you for your reply

[gortex2]

OK. First and foremost GMRS is not 70cm. Thats the HAM UHF band. Many commercial repeaters can be found in the UHF 460 Mhz range. They are cheap. 

LMR400 is not the best cable for duplex operations. Many posts on here about the shielding and issues you will find with duplex operation.

Ed Fong antenna is also not a good antenna for a repeater. Again many posts on here but its basically a put together antenna made by a ham. You want a worthwhile repeater buy a antenna made for the use case. 

Duplexer jumpers should be a well shieled jumper with the proper connectors on each end. Short as possible to the radio. 12" are the standard most of the time. RG142 is a good cable to use. 

Again manage your expectations. The setup you are proposing may give you 5 miles. I guess if its on a 6000' mountain it may talk further. No idea. Do a search on the site. Many folks have started with cheap setups like this and found it didn't do what they want. Either they disappear or spend the funds to do it correctly. 

[WRUE538]

70cm was simply the closest chart I could find to 462-470 to find cable loss. According to anything I could find, LMR400 had the least loss. It's much less flexible (roughly .400") but more suitable for our rough sunlight than RG142. I looked for a better 50w solution than twin Wouxons. I only found 1 used Motorola unit around $350 which is an old end of life product. Yes, I'm around 6800'

https://www.data-alliance.net/blog/lmr400-coax-specifications-characteristics-lowest-signal-loss-in-its-class/

[KAF6045]

1 hour ago, WRUE538 said:

70cm was simply the closest chart I could find to 462-470 to find cable loss.

https://www.qsl.net/co8tw/Coax_Calculator.htm

Belden 9913 isn't far behind. 2.905 dB (25.612W at antenna from 50W, 100ft, perfect SWR)
LMR400 2.735 dB (26.638W, same input, length, SWR)

Lucky you can get away with a stock cable configuration. I paid $96 for a 2ft length of LMR400 with an N connector (for the window pass-through) and PL-259 for the diplexer (not duplexer, the TS-2000 has separate 2m and 70cm outputs that I had to combine to feed a dual band ground plane) Most of the cost is apparently labor to install custom connector mix, as the 75 ft LMR400 (with same connector mix) was $191, just twice the 2ft -- so the cable was the cheap part

 

[marcspaz]

 

I have a portable 50w repeater system that runs on 14vdc.  I use 2 antennas and 2 masts because they are easier and cheaper to transport than a $1,500 duplexer the size of a cooler.  I can endorse that as a method, no problem.  It's not ideal for a permanent station, but it 100% works.

 

That said, 20 feet is not far enough apart.  Even with as little as 5 watts, I get desensing unless the antennas are at least 150 feet apart.  You may not realize the receive is being desensed until you turn the transmitter off.  When I run the full 50 watts, not only are they 150 feet apart, but I put the transmit antenna at a lower elevation than the base of the receive antenna, to help reduce the desensing even more. 

 

Hmmm  I wonder if I should try stacking so the two verticals so they are in the nulls of each other.  I might test that.

 

  

8 hours ago, gortex2 said:

RG142 is a good cable to use. 

 

Man... I know RG142 has a high frequency rating, but I would never use it for GMRS or lower portions of UHF.  Real-world, my RG142 had more than 8.5dB of line loss for 100 feet, compared to 4.1dB with RG8x and 3.4 with LMR400.  Have you done any metered loss testing?

[JeepCrawler98]

11 hours ago, marcspaz said:

 

I have a portable 50w repeater system that runs on 14vdc.  I use 2 antennas and 2 masts because they are easier and cheaper to transport than a $1,500 duplexer the size of a cooler.  I can endorse that as a method, no problem.  It's not ideal for a permanent station, but it 100% works.

 

That said, 20 feet is not far enough apart.  Even with as little as 5 watts, I get desensing unless the antennas are at least 150 feet apart.  You may not realize the receive is being desensed until you turn the transmitter off.  When I run the full 50 watts, not only are they 150 feet apart, but I put the transmit antenna at a lower elevation than the base of the receive antenna, to help reduce the desensing even more. 

 

Hmmm  I wonder if I should try stacking so the two verticals so they are in the nulls of each other.  I might test that.

 

  

 

Man... I know RG142 has a high frequency rating, but I would never use it for GMRS or lower portions of UHF.  Real-world, my RG142 had more than 8.5dB of line loss for 100 feet, compared to 4.1dB with RG8x and 3.4 with LMR400.  Have you done any metered loss testing?

This page has some useful charts when going the physical separation route: http://www.repeater-builder.com/antenna/separation.html

A good duplexer should give you more than 80dB of isolation (100dB+ if you get a really good one and take forever to tune it).

[marcspaz]

@JeepCrawler98 that was a fun read.  Thanks!

 

[wrci350]

19 hours ago, KAF6045 said:

Most of the cost is apparently labor to install custom connector mix, as the 75 ft LMR400 (with same connector mix) was $191, just twice the 2ft -- so the cable was the cheap part 

Not sure where you got your cables but $96 for a two-foot cable is way out of line IMHO.

I just priced a two-foot LMR400UF cable at TheAntennaFarm*.  The cable itself is $1.68 a foot, so a whopping $3.36.  It's only $10 to have both connectors installed with heat wrap, so it's not the labor.  The cost is in the connectors themselves.  For an N male you can pay $43.37, or you can pay $5.46.  PL-259?  $10.48 or $3.14.  Even if you opted for the most expensive connectors you'd be looking at $70, not almost $100. 

 

*Highly recommended, BTW.

[wrci350]

20 hours ago, WRUE538 said:

LMR400 had the least loss. It's much less flexible (roughly .400")

Take a look at LMR400UF (the UF is for UltraFlexible).  A bit more loss, but very easy to work with.

[WRUE538]

Greatly appreciated input guys. I've learned a lot. LMR400 not a good choice if using a Single input/receive. I'd read at least 3 meters spread was sufficient. Apparently not the case. As marcspaz has witnessed. The Wouxuns come with a 25' pairing cable. I'm guessing using some of that distance also would help with desense, according to Randy (notarubicon) I love his common sense approach and "some" of his humor. Leaning towards a Fumei 50W dup, and taking the time to tune well. I've heard nothing but good about that one, nothing but bad about the 30w version. Currently building hamo cans w/Wouxun KG-XS20G. We'll probably have repeater issues thought thru before winter. Thanks again!

[KAF6045]

17 hours ago, wrci350 said:

Not sure where you got your cables but $96 for a two-foot cable is way out of line IMHO.

I just priced a two-foot LMR400UF cable at TheAntennaFarm*.  The cable itself is $1.68 a foot, so a whopping $3.36.  It's only $10 to have both connectors installed with heat wrap, so it's not the labor.  The cost is in the connectors themselves.  For an N male you can pay $43.37, or you can pay $5.46.  PL-259?  $10.48 or $3.14.  Even if you opted for the most expensive connectors you'd be looking at $70, not almost $100. 

 

*Highly recommended, BTW.

DX Engineering...