Topics

1:1 Choke Location on Beam Installation

Brian Gohl - AI6US
 

Prepping to permanently install the hexbeam at my home QTH this weekend and researching the install location of the 1:1 line isolator choke.

I understand that it is recommended to install the choke at the antenna feedpoint, but I am looking for guidance on instead placing the choke inline at an alternative location below the flexible coax rotor loop. Are the any issues to consider if  installing the inline choke at the top of the tower then running a flexible coax approximately 6 feet up to the hexbeam feedpoint? Working 6m - 20m bands on the hexbeam, would the jumper length between the choke and hexbeam feedpoint have any influence? Would a 6' vs a 6" jumper have any effect on interaction (common mode???) if a VHF/UHF antenna were to be co-located on the tower?

Mounting the hexbeam's 1:1 balun choke in this location will reduce the number of connectors on the tower to transition from the flexible rotor loop coax to the stiffer LMR400. The choke would be able to dual purpose as the coax transition barrel.

Thanks for any advice!
--
Brian- AI6US

Dennis - WU6X
 

I think it won't make much difference, but check K9YC's page at: http://k9yc.com/publish.htm who is doctor of all things ferrite. I'm sure you will find something there related to this subject.
73,
Dennis, WU6X


From: sfarc@w6ek.groups.io <sfarc@w6ek.groups.io> on behalf of Brian Gohl - AI6US <ai6us@...>
Sent: Wednesday, February 19, 2020 9:42 AM
To: sfarc@w6ek.groups.io <sfarc@w6ek.groups.io>
Subject: [from W6EK Groups.io] 1:1 Choke Location on Beam Installation
 
Prepping to permanently install the hexbeam at my home QTH this weekend and researching the install location of the 1:1 line isolator choke.

I understand that it is recommended to install the choke at the antenna feedpoint, but I am looking for guidance on instead placing the choke inline at an alternative location below the flexible coax rotor loop. Are the any issues to consider if  installing the inline choke at the top of the tower then running a flexible coax approximately 6 feet up to the hexbeam feedpoint? Working 6m - 20m bands on the hexbeam, would the jumper length between the choke and hexbeam feedpoint have any influence? Would a 6' vs a 6" jumper have any effect on interaction (common mode???) if a VHF/UHF antenna were to be co-located on the tower?

Mounting the hexbeam's 1:1 balun choke in this location will reduce the number of connectors on the tower to transition from the flexible rotor loop coax to the stiffer LMR400. The choke would be able to dual purpose as the coax transition barrel.

Thanks for any advice!
--
Brian- AI6US

--
Dennis - WU6X

Jef - N5JEF
 

Brian -

The choke really should be at the antenna feed point. It is intended to present a relatively high impedance at that point, and minimize common mode radiation from the feedline. 

What's your highest band on that beam?

As you know, a quarter wave transmission line transforms an open circuit to a short, and vice versa. 

So, at 10 meters your 6 feet of coax (remember also consider the velocity factor) will be roughly a quarter wave and your choke will have roughly the opposite of its intended effect. At 20 meters, roughly 1/8 wave, so the transformation, as seen at the feed point, will be somewhere in between, and certainly not the high impedance as designed. 

That said, ham radio xperience tells us that antenna theory is all magic anyway, and you'll still make contacts proving it works great. 

So FWIW, have fun. 

-Jef N5JEF 



On Wed, Feb 19, 2020, 16:57 Dennis - WU6X <wu6x@...> wrote:
I think it won't make much difference, but check K9YC's page at: http://k9yc.com/publish.htm who is doctor of all things ferrite. I'm sure you will find something there related to this subject.
73,
Dennis, WU6X


From: sfarc@w6ek.groups.io <sfarc@w6ek.groups.io> on behalf of Brian Gohl - AI6US <ai6us@...>
Sent: Wednesday, February 19, 2020 9:42 AM
To: sfarc@w6ek.groups.io <sfarc@w6ek.groups.io>
Subject: [from W6EK Groups.io] 1:1 Choke Location on Beam Installation
 
Prepping to permanently install the hexbeam at my home QTH this weekend and researching the install location of the 1:1 line isolator choke.

I understand that it is recommended to install the choke at the antenna feedpoint, but I am looking for guidance on instead placing the choke inline at an alternative location below the flexible coax rotor loop. Are the any issues to consider if  installing the inline choke at the top of the tower then running a flexible coax approximately 6 feet up to the hexbeam feedpoint? Working 6m - 20m bands on the hexbeam, would the jumper length between the choke and hexbeam feedpoint have any influence? Would a 6' vs a 6" jumper have any effect on interaction (common mode???) if a VHF/UHF antenna were to be co-located on the tower?

Mounting the hexbeam's 1:1 balun choke in this location will reduce the number of connectors on the tower to transition from the flexible rotor loop coax to the stiffer LMR400. The choke would be able to dual purpose as the coax transition barrel.

Thanks for any advice!
--
Brian- AI6US

--
Dennis - WU6X

Brian Gohl - AI6US
 

Thank you for your replies. Great information and references.

Wow!!! The K9YC link has an incredible trove of information. I think that I will be referencing that site often! Thanks Dennis!

Thank you Jef for the feedline length explanation. The hexbeam is 6m-20m, but in testing the 6m band, it is very inefficient and will likely not be used much. Your examples certainly confirms the concerns regarding the interactions of a radiating jumper on other frequencies. I will be using a 2 foot LMR-400 jumper to place the isolation choke weight near the bottom of the hexbeam center post, use a 3 foot LMR-240 UF coax jumper for the rotor loop then a barrel to transition to the LMR400 run to the shack.

Thanks again!
--
Brian- AI6US