2
$\begingroup$

I am working on putting together a 40 meter delta loop using a 1/4 wavelength of RG-6 as the matching section. However I'm unsure of where to create my feedpoint choke.

Do I coil up the RG6 right at the feedpoint of the antenna itself? Or do I coil up the 50 ohm RG8x at the point where it feeds into the 75 ohm matching section?

EDIT: My choke consists of 8 turns of coax through a mix 31 big clamp on bead.

enter image description here

$\endgroup$
5
$\begingroup$

The choke should go at the transition between unbalanced and balanced, which is where the RG-6 meets the ends of the loop wire.

You talk of coiling coax: I suggest you coil it around a ferrite core and not just air. The trouble with air-core "chokes" is they have no resistance, only reactance, in common-mode. Depending on what the common-mode impedance is in the rest of the antenna system, adding this reactance might actually tune the common-mode to resonance, making things worse. A good choke has a substantially resistive impedance: this is guaranteed to raise the overall common-mode impedance in all cases. See additional explanation by G3TXQ.

$\endgroup$
  • 1
    $\begingroup$ I presume that you mean that Paul should use a ferrite-core balun, rather than actually wrapping coax around a piece of ferrite. The minimum bend radius of many varieties of coax would require quite a big piece of ferrite ;) $\endgroup$ – rclocher3 Aug 24 '18 at 18:59
  • $\begingroup$ Comment moved to answer. $\endgroup$ – Mike Waters Aug 24 '18 at 19:10
  • 1
    $\begingroup$ @MikeWaters I was thinking of the 4 inch (10 cm) minimum bend radius of LMR-400, but then I remembered this discussion, which concluded that the specification for "bend radius: repeated" of 1 inch (2.5 cm) can be used for a one-time installation. $\endgroup$ – rclocher3 Aug 24 '18 at 19:35
  • 1
    $\begingroup$ @rclocher3 Also, the coax doesn't need to be pulled tight around the core. $\endgroup$ – Phil Frost - W8II Aug 24 '18 at 19:56
  • 1
    $\begingroup$ Edited my post to describe my choke solution. $\endgroup$ – Paul Aug 25 '18 at 0:34
4
$\begingroup$

Forget about air-core chokes. G3TXQ's page that Phil links to in his excellent answer here shows just how much superior ferrite chokes are than air core choke baluns.

Please visit k9yc.com and download rfi-ham.pdf. Jim Brown there on page 36 recommends a choke balun made by wrapping 8 turns of the coax at the antenna feedpoint through four stacked 2.4" diameter #31 ferrite cores. They cost about $7 from http://www.mouser.com. And don't wrap the RG-6 too tight! That PDF tells how to properly wrap it.

Below is a similar choke balun that I made, at the base of my 160m inverted-L. Those are the exact Fair-rite #31 cores I mentioned above, with RG-6 coax. You don't need to make the loops as big as I did.

W0BTU 160m choke balun

(For 1.8 MHz, it really should have had more turns and cores, but it worked very well nonetheless.) I used this antenna on 160 through 20 meters, with great results.

$\endgroup$
  • 1
    $\begingroup$ I added my choke to the original post. Will that suffice? $\endgroup$ – Paul Aug 25 '18 at 0:35
  • $\begingroup$ @Paul Very good! I edited your post and changed your link to show the actual photo there. $\endgroup$ – Mike Waters Aug 25 '18 at 0:55
  • $\begingroup$ @Paul Ok, you're using RG8-X and RG-6. Which is the light-colored coax? And which type is the dark colored coax? Most importantly, where is the antenna connected in this photo? Please go back and read Phil's answer here. :-) $\endgroup$ – Mike Waters Aug 25 '18 at 1:00
  • $\begingroup$ That came from my old antenna. It is just an example of how I plan on making the choke. It sounds like I should do it with the RG-6 at the feedpoint of the delta loop. $\endgroup$ – Paul Aug 25 '18 at 2:00
  • $\begingroup$ @Paul Understood! You are correct. $\endgroup$ – Mike Waters Aug 25 '18 at 2:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.