Homemade Coax Feedline Choke

[poppafred]

I am going to put up a Husler 4-BTV when it arrives.

Does anyone have any experience building their own feedline choke?

I cannot afford to buy the commercial feedline choke and mounting hardware right now, that will have to come later. Need to buy direct bury coax first anyway.

The 4-BTV manual describes how important it is to to have a feedline choke in line and how to build one using coax, so I sat down with a 5# coffee can (6" diameter) and a 30' piece of RG-8X and coiled up a choke. It took almost all of the 30' to make it.

The thing is, would it harm anything to use heavy duty all weather duck tape?

It holds the coils very well, it is supposed to tolerate exposure to the weather well and I had a roll already. I can undo it if I have to but will the duck tape harm anything?

 

[LtDoc]

There's nothing super critical about a coaxial choke. Using ductape to keep it together should work just fine. That choke is to 'choke off' unwanted signals/charges from the outside of the coax's braid. Burying that coax does that too, so that's to you benefit also.
The signals/charges on the inside of that coax is the good stuff, the signal/charges on the outside isn't doing any good at all, so should be gotten rid of. Big difference between the two.
- 'Doc

 

[prcguy]

The chokes made by coiling up coax are really not that effective and are narrow band. If you optimize it for 40 or 80m their pretty useless on 10m, etc.

For a few dollars you can get an FT--240-43 torroid core from Amidon and wrap 10 turns of coax through it (small coax like RG58 or 8X) and it makes a very effective wide band choke.
prcguy

 

[LtDoc]

Got an old TV sitting around? If it has a gob of small wire around the neck of the picture tube, take a look under that wire. There's a graphite core which works very nicely as a 'torroid' for chokes. Sort of bend that choke to fit the core...
- 'Doc

 

[kb0nly]

I have a 6-BTV and made a coax choke of RG-8 sized coax, ten turns, as suggested in the install manual and thats right at the feedpoint of the antenna. I have no problems whatsoever on all bands. The tuner touches up 80 a smidge when i go out to the top or bottom of the band, the rest of the bands are pretty flat. I have been thinking of adding on bands to it but haven't got that far yet.

I got mine in need of repair and did a writeup on its repair.

Huster BTV Repair

I need to get some pictures of the complete install but its not entirely complete yet! I have a ground rod in at its base but i haven't got the radials done yet, i plan on adding them just havent had the time and money yet. The reports with it so far are very favorable though and its a much quieter antenna then my dipole.

 

[poppafred]

I have always tried to take shortcuts on antennas. Fudge a little here or there, you know. This time I am doing it by the book. I ordered the 4-BTV package from DX Engineering with the tilt mount and the radial wire plate. Even ordered an extra 500 ft of wire for the radials. I am going to put in 56 radials, 14 wires for each of the 10, 15, 20 and 40m bands. DX Engineering has a performance guide you can download and it recommends the feedline current choke they manufacture and sell (of course). Cannot afford that right now, gotta save the pennies for some new, direct bury coax first. Eventually, I will get their choke but right now, I'll settle for the coax choke.

Thanks for the input, folks.

 

[prcguy]

If you have an antenna analyzer available like the MFJ 259 you can get an idea of how effective your feed line choke is.

Take several inches of wire and short the center of the antenna socket to the case ground and you will measure an infinite match thoughout the HF band and 0 ohms impedance because it's a short. This is a reference measurement that shows your meter is working.

Then connect the outer shield of your feed line choke in the same manner and you can measure the appxoximate choking impedance.

A coil of coax might be off the scale above 500ohms at some frequency but it will be under 100ohms across many frequencies and close to a short or no choking impedance at other freqs. It's really not a very good broad band choke.

Test a simple home made choke with 10 turns of coax around an FT-240 ferrite core with a #43 or #61 or similar mix and the choking impedance will be off the MFJ analyzer scale over most of the HF band until you get down below 80m where is still around 500ohms.
prcguy

I have a 6-BTV and made a coax choke of RG-8 sized coax, ten turns, as suggested in the install manual and thats right at the feedpoint of the antenna. I have no problems whatsoever on all bands. The tuner touches up 80 a smidge when i go out to the top or bottom of the band, the rest of the bands are pretty flat. I have been thinking of adding on bands to it but haven't got that far yet.

I got mine in need of repair and did a writeup on its repair.

Huster BTV Repair

I need to get some pictures of the complete install but its not entirely complete yet! I have a ground rod in at its base but i haven't got the radials done yet, i plan on adding them just havent had the time and money yet. The reports with it so far are very favorable though and its a much quieter antenna then my dipole.

 

[LtDoc]

A coaxial choke is just an inductor which has a fairely high impedance at particular frequencies. They are frequency specific, or maybe 'band specific' would be a better way of putting it. Being 'band specific' that means that they are never going to be all that wide banded in terms of 'bands'. Frequencies yes, bands no, to big'a chunk, the impedance lowers the further away from a specific frequency (or band). If you can make a choke that works well for one band, don't expect it to work well on several band, harmonically related or not. To be very widely affective the amount of impedance has to be variable, meaning the size of the choke has to be variable. Not very likely unless you go to a lot of trouble making it so (variable inductance or inductor made up of coax cable).
Or, instead of having just one choke, how about having several, one for each band? Certainly possible, but far from a simple, single choke for all bands in simplicity.
- 'Doc

 

[990adv]

How would one go about making a band specific choke? What is the formula? I would imagine type of coax/velocity factor would also play a role. Lower frequency would have a larger loop, correct?

 

[LtDoc]

That choke will only affect the outside of the feed line, not the inside. So, the velocity factor of the 'inside' of the coax doesn't have any affect. What does matter is the amount of inductance provided by that choke and that inductance's affect at the frequency of use. Since inductance's affect (impeding the signal) at the frequency of use is what's wanted, select that the inductance that provides the most "stopping power" (impedance) at the desired frequency range.
There are two formulas that can get you to where you want. One involves defining inductance and the frequency of use, and the other tells you how much inductance a particular size of coil produces. I look in the ARRL handbook for those formulas but they aren't secret by any means. Look'em up!
- 'Doc