Comet 712 Coax Loop

[cateyetech]

Good Morning

 

I have a question about our Comet CA-712EFC antenna.

 

Here are the supplied instructions

 

They say to make a loop or two with the coax.

I was wandering the reason for this?

Also has anyone using this antenna added the loop?

Will it affect the propagation pattern?

 

We did add the loop, app 18-24" diameter 

and down from the antenna app 24"

 

 

We are running a desktop base/repeater @40W 

with a internal duplexer

70' Commscope LFD4-50A coax

Comet 712  with the base at 25' above the ground (& we live on a good hill)

 

We are getting good clear communications across down town (app 7miles) to a Baofeng GMRS V1 HT

so I think our setup is all good.

 

Any thoughts on this loop?

Thanks Charlie

 

 

 

 

 

[cateyetech]

I just noticed this was my first post on this forum.

 

So Hello Fellow GMRS operators & enthusiast

 

WRAN 763  Charlie Mc here in NW Arkansas

 

Been 11m for a while, FRS for 3 or so years with the grandkids :)

Sold ALL! my 11m equipment and went GMRS this year.

 

I'm a very slow keyboard operator, so I won't post much.

But I read a lot. 

Really it better that way.

 

Thank Ya'll for all the great GMRS info!!!

 

Charlie

[JohnE]

its a service loop.

[cateyetech]

its a service loop.

 

Thank you JohnE 

That makes since

That's also something I would have never thought of

[SteveC7010]

To add to JohnE's comment, any antenna install should sufficient slack at antenna AND radio ends for this. Base antennas typically get a loop as shown. Mobile antennas usually just get some slack. A fixed loop isn't necessary there and at the radio. The same logic applies to any other wiring at the radio

[n4gix]

The loop serves as an RF choke, and acts to reduce common mode current. This will assist in reducing the tendency of the coax to act as an active part of the antenna, which affects both transmission and reception.

 

If you really want to get into the technical explanation, see this PDF article: http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

 

As I use 1/2" heliax, I can't create a loop, but then heliax obviates the need for a common mode choke anyway.

 

To see many images of such coax chokes in action, see here:

https://www.bing.com/images/search?q=coax+loop+choke&id=2F362EF35CC88DA2316C4C069F5E278D0AA1C188&FORM=IQFRBA

[cateyetech]

Thanks SteveC7010 for the heads up 

I leave some slack in the coax on all my stuff, maybe because I'm not that neat

 

Thanks n4gix

I thought that there might be a reason they said to add a loop.

We ran a choke on our Solarcon A-99 , we live right off I-40 at the edge of town, a mile away from two big truck stops 

It's a spewing RF nightmare around here.

 

I tried to read the pdf you linked, but WOW!!! way over my skillset, but thanks for the info.

 

As for the physical loop, before unrolling the coax I slowly formed the loop then straightened out  the rest.

I really didn't want to bend this expensive (to us) new coax but the antenna manufacture knows WAY more than a radio loven hillbilly  

so I did it anyway.

 

again thanks for the info ya'll

[Durake]

This is something I didn't know either, now I understand. I'll do this sometime, it'll make the install in the car look a lot nicer in the back. 

[dwmitchell61]

On 2/23/2018 at 9:23 AM, n4gix said:

The loop serves as an RF choke, and acts to reduce common mode current. This will assist in reducing the tendency of the coax to act as an active part of the antenna, which affects both transmission and reception.

 

If you really want to get into the technical explanation, see this PDF article: http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

 

As I use 1/2" heliax, I can't create a loop, but then heliax obviates the need for a common mode choke anyway.

 

To see many images of such coax chokes in action, see here:

https://www.bing.com/images/search?q=coax+loop+choke&id=2F362EF35CC88DA2316C4C069F5E278D0AA1C188&FORM=IQFRBA

I am curious as to how the heliax obviates any common mode current on the outer skin of the shield.

Maybe I am misunderstanding the cause of common mode current on the feedlines?

I thought it was due to imbalance in the antenna system.

The PPT presentation you linked to above, on Page 6, states"

"– Coax is not a part of these imbalances"

So, I am wondering how your expensive heliax transmission line solves the issue. I have a lot to learn and don't know it all and am open to some mentoring from an "Elmer"

It would seem that using ferrite beads of a proper mix rather than the old, trusty "ugly balun" method would be more effective at higher frequencies like GMRS.

 

Thank you and 73

Dave, KJ7WUZ/WRHW845