Reusing existing (TV?) antennas for base station and/or repeater

[WRZY540]

My recently purchased house is situated on top of a small hill and has an antenna setup (from a former owner, no longer used) with both yagi and dish antennas at the apex of the roof. With no nearby homes or trees to interfere, I think this is an ideal spot for a GMRS repeater antenna but have to wonder if the existing antennas could be repurposed. This would be ideal since they are already grounded and coax is wired through the walls to locations where the repeater could be installed.

From what I have read elsewhere, seems the answer is that I need to remove these and  install something tuned for GMRS, but I know little about antenna theory so checking with this group. 

 

[SteveShannon]

The dish is a satellite antenna with an LNB as part of the assembly. It has no practical possibility of being repurposed to 462-467 MHz frequencies. 
The one you called a Yagi is really a log periodic. Log periodic antennas are very wideband. It might be repurposed, but without an antenna analyzer or VNA it might be difficult. It’s very directional so it would not be ideal as a repeater antenna unless you wish to limit your communications to a narrow field. But if you’re pointing it at a distant repeater it could be okay. 
Also the coax is 75 ohm rather than 50 ohm. That will limit how low you can get the SWR, but it’s not the kiss of death. 
You will need an F to UHF adapter though. They’re easy enough to obtain. 
I would hook up to the log periodic antenna, point it at a repeater and see if you receive. If you do, try transmitting on low power. Even if you have a high SWR by using low power you help protect your final transistors from damage. 

[BoxCar]

The dish can be reworked as a slot antenna giving you a directional vertically polarized antenna. There is a book on Amazon you can get that provides the details on building the slot antenna.

[Lscott]

3 hours ago, Sshannon said:

I would use the coax. It’s not perfect but it’s not terrible. It’ll prevent you from getting an swr better than 1.5:1, but most people are satisfied with that. 

You forgot about the necessity of cutting off those crummy F-connectors, replacing them with “N” type or PL-259, if you can find any that fit that coax size, or finding the correct adapter. 
 
I think you’re better off installing the correct antenna and coax and save yourself the extra hassle.

[SteveShannon]

41 minutes ago, Lscott said:

You forgot about the necessity of cutting off those crummy F-connectors, replacing them with “N” type or PL-259, if you can find any that fit that coax size, or finding the correct adapter. 
 
I think you’re better off installing the correct antenna and coax and save yourself the extra hassle.

No, I said: 

 

4 hours ago, WRZY540 said:

You will need an F to UHF adapter though. They’re easy enough to obtain. 

Such as this: https://www.americanradiosupply.com/uhf-male-to-f-female-coaxial-adapter-connector/?gclid=Cj0KCQiA1rSsBhDHARIsANB4EJaXnN6NDUi0pnfCrPC7u1jPPlHuNPlbzokazzddein9TZ6qF3Rgw_0aAjaEEALw_wcB

 

[SteveShannon]

6 minutes ago, GreggInFL said:

So, I could probably use the coax that DirecTV installed 29 years ago?

You can, but it will never be the best option. You have to weigh the advantages against the disadvantages.

How long is it? Which leads to calculating the losses.  RG-6 loses nearly 5 dB in 100 feet on GMRS frequencies.

How difficult would it be to replace? If it’s easy, and the cable is long, go for something better.  If it’s short and difficult to replace then I would at least try it.

Basically I would try it and see first.

 

[WRZY540]

3 hours ago, Sshannon said:

The dish is a satellite antenna with an LNB as part of the assembly. It has no practical possibility of being repurposed to 462-467 MHz frequencies. 
The one you called a Yagi is really a log periodic. Log periodic antennas are very wideband. It might be repurposed, but without an antenna analyzer or VNA it might be difficult. It’s very directional so it would not be ideal as a repeater antenna unless you wish to limit your communications to a narrow field. But if you’re pointing it at a distant repeater it could be okay. 
Also the coax is 75 ohm rather than 50 ohm. That will limit how low you can get the SWR, but it’s not the kiss of death. 
You will need an F to UHF adapter though. They’re easy enough to obtain. 
I would hook up to the log periodic antenna, point it at a repeater and see if you receive. If you do, try transmitting on low power. Even if you have a high SWR by using low power you help protect your final transistors from damage. 

Thanks, this is very helpful. Given the low cost of a j pole antenna, I was thinking the best alternative would be to remove the dish and the log periodic from the mount, connect a j pole and reuse the ground. I wasn’t sure if the coax would be reusable but it sounds like no. 

[SteveShannon]

7 minutes ago, WRZY540 said:

Thanks, this is very helpful. Given the low cost of a j pole antenna, I was thinking the best alternative would be to remove the dish the log periodic from the mount, connect a j pole and reuse the ground. I wasn’t sure if the coax would be reusable it sounds like no. 

I would use the coax. It’s not perfect but it’s not terrible. It’ll prevent you from getting an swr better than 1.5:1, but most people are satisfied with that. 

[WRYZ926]

From the looks of things in the photo, you could remove the dish and then slide the pole up to get some more height. You can try reusing the existing cable with adaptors. But definitely check the SWR with a NanoVNA or other antenna analyzer. Worst case would be to run a better coax cable such as LMR400 to a GMRS tuned antenna. 

[Lscott]

4 hours ago, Sshannon said:

No, I said: 

 

Such as this: https://www.americanradiosupply.com/uhf-male-to-f-female-coaxial-adapter-connector/?gclid=Cj0KCQiA1rSsBhDHARIsANB4EJaXnN6NDUi0pnfCrPC7u1jPPlHuNPlbzokazzddein9TZ6qF3Rgw_0aAjaEEALw_wcB

 

Ok. 

[GreggInFL]

On 12/28/2023 at 1:14 PM, Sshannon said:

I would use the coax. It’s not perfect but it’s not terrible. It’ll prevent you from getting an swr better than 1.5:1, but most people are satisfied with that. 

So, I could probably use the coax that DirecTV installed 29 years ago?

[GreggInFL]

Thanks, Steve.  Makes sense.  Didn't realize the gain loss was so high.  I'm probably at 100' end-to-end.