70 cm Antenna System

[bill4long]

"It is brought into the repeater with 6' of RG-213."

That length is longer than a wavelength. Try swapping this out for some Heliax ASAP. Here's why:

For RG-213, the center conductor and shield have a different metal composition. This is a big no-no for in-band duplexing, especially at UHF frequencies. (It doesn't matter with cross-band duplexing.) Spurious frequencies can be generated that will desense the receiver. Heliax and hardline have the same center/shield metal composition and do not have this phenomenon. This is a well-known issue.

The Comet antenna should be fine, it's got about 7 dbd gain, which ain't bad, unless there is a defect in that particular unit. I have used equivalent antennas with mobile duplexers with excellent performance on UHF. Sidebar: use silicon tape around the N-connector after you connect the coax "permanently."

 

[kayn1n32008]

...For RG-213, the center conductor and shield have a different metal composition...

What two metals?

Edit:

LMR-400 is different metals, but any time I have played with RG-213 it was copper and copper.

 

[bill4long]

LMR-400 is different metals, but any time I have played with RG-213 it was copper and copper.

The copper braid shield is slightly tinned. You cannot tell by looking. But it's there.

 

[N4GIX]

"It is brought into the repeater with 6' of RG-213."

That length is longer than a wavelength. Try swapping this out for some Heliax ASAP.
<snipped for brevity>
Sidebar: use silicon tape around the N-connector after you connect the coax "permanently."

Thanks for the comments. As stated several times I will be replacing the RG-213 (which is 20+ years old anyway) with 1/4" Superflex as soon as it's delivered.

Meanwhile, I seem to have overestimated the length I needed for my heliax by about 12'. Currently the excess is coiled into four large loops. Now I'm wondering if that's acting like a choke.

I'm about to answer the question by uncoiling the loops and passing the connector through my window sash directly to the repeater, bypassing the lightning arrestor and RG-213 altogether.

BTW, all connectors are silicon double-wrapped tightly, and then double-wrapped with quality electrical tape.

 

[WA0CBW]

I would go with RG-214/U instead of the 1/4" superflex.
BB

 

[kayn1n32008]

Meanwhile, I seem to have overestimated the length I needed for my heliax by about 12'. Currently the excess is coiled into four large loops. Now I'm wondering if that's acting like a choke.

Yes it can act like a choke. Saw the effects first hand 'testing' a permanent RTK reference station before it was installed. The person responsible did not want to unspool the 50-ish metres of 1/2" hardline on the spool. He was lucky to have not burned out the transmitter. Even though it is about a 50% duty cycle the transmitter was smokin hot after only about 30 minutes.

 

[N4GIX]

Yes it can act like a choke. Saw the effects first hand 'testing' a permanent RTK reference station before it was installed. The person responsible did not want to unspool the 50-ish metres of 1/2" hardline on the spool. He was lucky to have not burned out the transmitter. Even though it is about a 50% duty cycle the transmitter was smokin hot after only about 30 minutes.

50ish meters on a spool is a order of magnitude greater than 12' in a four loop coil. Keep in mind too that such 'chokes' can actually help balance out an antenna system, although they are typically at the base of the antenna.

Having now un-coiled the heliax and run it straight into the repeater, I now note that the impedance is radically different than when the 'choke' coil was present. Note how it's increased from the ~50 ohms previously recorded:

 

[nd5y]

Coiling up coax CANNOT affect the signal on the inside.
If that changes anything it's because your coax is radiating beacuse the antenna isn't properly decoupled.

 

[N4GIX]

I would go with RG-214/U instead of the 1/4" superflex.
BB

Well, it certainly is less expensive. I've placed an order for a 6' length today, along with some good N-Male compression connectors.

 

[bill4long]

RG-214 is good for your short run, it has silver plated copper for both inner conductor and shield.

 

[Project25_MASTR]

It is a Comet antenna... Regardless as to whether it is tuned for the ham band or the commercial band the antenna quality is of cheap hammy garbage. Comet is Comet is Comet. It's junk.



You are having desense problems. What kind of jumpers are between your duplexer and the repeater. I would start looking there.



dBd = gain over a 'd'ipole.




If I remember correctly, you were getting 70-ish dB isolation... In retrospect @5MHz separation you should be able to get more. First thing would be to get rid of the Rg-213 and use something 100% shield or double shielded.

Pretty sure my Cellwaves were all about 90 dB.

Sent from my SM-T350 using Tapatalk

 

[N4GIX]

Two updates to the situation for today. I received an email from Ron K. at Bridgecom who said that if I ship the repeater back to them in Missouri, he'll replace the duplexer with a new one from their new vendor, and also check out the entire system again, under warranty of course.

But first, last night I had another long QSO with my friend WQVY669 from his work location near the intersection of Columbus Ave. and Indianapolis Blvd. This is about 4 airline miles from me. I could hear him faintly on the repeater, but not reliably. He said he was going to go to 462.550 simplex.

On a whim I got up and wandered back into the front office/hamshack and dialed that into my TYT TH-7800 and tossed out a call.

Imagine my surprise when I got his reply at absolutely full quieting, with a solid four of eight bars on my digital scale! So, there are now two possibilities that occur to me since the only differences are:

1.antenna system
2.radio receiver

The TH-7800 is connected to my "old" dual-band antenna at roughly the same height as my new Comet 70cm antenna. I have a new Workman dual-band antenna and coax up on a chimney mount and have now snaked that cable through the window and have it connected to the repeater (after having swept it first of course!).

He'll be back at work late tonight and hopefully we can run some tests to compare the repeater's receiver with my TYT TH-7800's receiver.

 

[Project25_MASTR]

Two updates to the situation for today. I received an email from Ron K. at Bridgecom who said that if I ship the repeater back to them in Missouri, he'll replace the duplexer with a new one from their new vendor, and also check out the entire system again, under warranty of course.

But first, last night I had another long QSO with my friend WQVY669 from his work location near the intersection of Columbus Ave. and Indianapolis Blvd. This is about 4 airline miles from me. I could hear him faintly on the repeater, but not reliably. He said he was going to go to 462.550 simplex.

On a whim I got up and wandered back into the front office/hamshack and dialed that into my TYT TH-7800 and tossed out a call.

Imagine my surprise when I got his reply at absolutely full quieting, with a solid four of eight bars on my digital scale! So, there are now two possibilities that occur to me since the only differences are:

1.antenna system
2.radio receiver

The TH-7800 is connected to my "old" dual-band antenna at roughly the same height as my new Comet 70cm antenna. I have a new Workman dual-band antenna and coax up on a chimney mount and have now snaked that cable through the window and have it connected to the repeater (after having swept it first of course!).

He'll be back at work late tonight and hopefully we can run some tests to compare the repeater's receiver with my TYT TH-7800's receiver.

Duplexer is still part of the antenna system. As you said, in station mode the repeater seems to receive fine. Still pointing at intermod from the duplexer or feedline.

 

[N4GIX]

Duplexer is still part of the antenna system. As you said, in station mode the repeater seems to receive fine. Still pointing at intermod from the duplexer or feedline.

There is no "duplexer" on my TH-7800 at all. Based on last night's testing the difference between the RSSI and audio on the TH-7800 and the repeater in "Base" mode was night and day.

I took a drive around this afternoon and found that the repeater's performance is pretty much the same using the 10' lower Workman antenna as the 10' higher Comet and the expensive heliax.

I am strongly suspecting that it's a duplexer issue *and* a really poor front end on the receiver.