Base A99 Antenna Feedline Question

[Riverman]

I often see chokes being used with this antenna in an attempt to prevent common mode currents on the feedline. At the same time, I've often read the A99 uses the feedline as the "other half" of the antenna.
If the choke stops the coax from "radiating", has an important segment of the antenna been eliminated? And its efficiency reduced?

 

[338_MtRushmore]

My a99 was grounded to a metal mast. A choke made no difference in my setup, but I never tried isolating it. I had lots of rfi if the swr was over 1.4 or so. Good luck with the thread

 

[fourstringburn]

At the same time, I've often read the A99 uses the feedline as the "other half" of the antenna

If that is so, then CMC would be out of control all the way down to the shack!

The A99 is a 1/2 wave end fed dipole so there is no missing length.

CMC can happen with many antennas for different reasons and typically 1:1 choke baluns are used to keep CMC from traveling down the feedline.

 

[Riverman]

The A99 is a 1/2 wave end fed dipole so there is no missing length.

That's why I am confused. I've been told time and again that "all end fed dipoles need counterpoise and that the feedline provides it."

The source most commonly quoted is this:
https://www.w8ji.com/end-fed_1_2_wave_matching_system_end feed.htm

 

[loosecannon]

If that is so, then CMC would be out of control all the way down to the shack!

The A99 is a 1/2 wave end fed dipole so there is no missing length.

CMC can happen with many antennas for different reasons and typically 1:1 choke baluns are used to keep CMC from traveling down the feedline.

according to their literature, the Antron 99 is a "half over a quarter wave mutual transductance tuned antenna".
https://www.manualslib.com/manual/892881/Solarcon-A-99.html

the part they don't elaborate on is that the first 9 feet or so of either the coax or mast is used as a "stub" for tuning.

LC

 

[Riverman]

according to their literature, the Antron 99 is a "half over a quarter wave mutual transductance tuned antenna".
https://www.manualslib.com/manual/892881/Solarcon-A-99.html

the part they don't elaborate on is that the first 9 feet or so of either the coax or mast is used as a "stub" for tuning.

LC

Thanks, LC.
Have heard that before. If it's so, wouldn't a choke or balun affect the "tuning"? Seems it would.

 

[loosecannon]

having never tried it, i can't say for 100% certain, but i would guess that if the antenna was mounted to a non-metallic structure, and a feedline choke was placed right at the feedpoint, that the tuning would be affected as well as other characteristics.

having read and being part of many discussions over the years about the antron 99, and having owned three of them, my opinion is that they perform very differently depending on how and where they are mounted.

it's like you have to take each antron install and determine which part of that install is the culprit of the interference or bad performance.

LC

 

[Riverman]

having never tried it, i can't say for 100% certain, but i would guess that if the antenna was mounted to a non-metallic structure, and a feedline choke was placed right at the feedpoint, that the tuning would be affected as well as other characteristics.

having read and being part of many discussions over the years about the antron 99, and having owned three of them, my opinion is that they perform very differently depending on how and where they are mounted.

it's like you have to take each antron install and determine which part of that install is the culprit of the interference or bad performance.

LC

Guess the thing to do is stick one on a pole, connect the coax and forget it.

 

[338_MtRushmore]

Throw it up there and see what happens. If you do have problems there are several things you can try, but it may not need any fixing.

 

[fourstringburn]

according to their literature, the Antron 99 is a "half over a quarter wave mutual transductance tuned antenna".
https://www.manualslib.com/manual/892881/Solarcon-A-99.html

the part they don't elaborate on is that the first 9 feet or so of either the coax or mast is used as a "stub" for tuning.

LC

No wonder why they are referred to as "bleed sticks". Then the choke balun would have to be placed 9 ft below the feedpoint.

This pic below is supposed to be what the A99 is: A shunt fed 1/2 wave element with tuning coils.

 

[fourstringburn]

Also if the 1/4 wave coax stub is needed, then it must not be placed near a metal mast or any other metallic objects. Stand off brackets would have to be used or rigid PVC pipe for the last 9 ft or so.

 

[loosecannon]

yeah FSB that little detail has been the cause of a lot of dissatisfied users over the years.

I have always told people to put the choke 9 feet down from the feedpoint on these antennas, but as long as it's attached to a metal mast at least 9 feet long, i would figure that choking at the feedpoint would be fine.

the other thing i try to get across to people in these "antenna choke" discussions is that they should be using ferrite beads over the coax instead of winding the coax into coils.

the guy who made that chart that EVERYONE references came out and said that the only reason he made it was to show how inferior the coil winding was when compared to using ferrite.

he said it annoyed him to no end that everyone was using that chart to determine how many and what diameter coils to use. LOL


for anyone interested, palomar engineers sells a kit for this for about 30 bucks that works excellent.
https://www.ebay.com/itm/Palomar-En...178347?hash=item2cca8386ab:g:CdEAAOSwLa9UYjFK

LC

 

[fourstringburn]

but as long as it's attached to a metal mast at least 9 feet long,

I would disagree here.

Since the coax is part of the radiating element, than this coax section should NOT be placed near or on metal if that's what you are implying.

Because the 9ft coax section is now part of the radiating element, it should not be placed on or near metal because it can detune the antenna, cause unnecessary reflections, and degrade it's overall performance.

Attaching the antenna to a metal mast will give it a D.C. ground, not an RF ground. You can always break your coax connection at ground level with polyphasers attached to a ground rod. This will give a more effective D.C. ground through your coax braid and provide static discharge protection. This is what I use on all my antennas.

for anyone interested, palomar engineers sells a kit for this for about 30 bucks that works excellent.

I like this choke kit but it seems too simple and not enough ferrite beads to be completely effective. If you tried it and it works, then great because that's what matters.

Comtek makes a choke using 100 mix 43 ferrites over a piece of RG-400. This one costs around $120 but it looks effective with all those beads. These are an effective alternative to coil wound chokes, possibly better.

 

[Riverman]

Because the 9ft coax section is now part of the radiating element, it should not be placed on or near metal because it can detune the antenna, cause unnecessary reflections, and degrade it's overall performance.

THAT explains what went wrong with the Procomm Patriot 12 antenna I once had. When I first put it in the air, the SWR was great, the needle barely moving. Then after tidying things up and taping the coax to the mast, I went inside to enjoy it only to find the SWR was sky HIGH. No amount of adjusting the tuning rings, adding hose clamps to the tuning rings (as some suggested), or trimming the top of the antenna helped. I eventually hack-sawed it into several pieces and dumped it.

In hindsight I remember now that when I first checked the SWR, the coax was pulled off to one side at approximately a 45 degree angle. And only when I taped it to the metal mast did things go wrong. The Patriot 12 is notorious for SWR problems and so am guessing it is extremely sensitive to the radiating portion of the coax being near metal.

Holy cow.

 

[Old_Crow]

I made a little choke by turning a 8" loop with 5 turns with the coax at the antenna feed point which helped my situation.