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I have been using a Par EndFedz Mk2 Trail Friendly 40/20/10 though using the matchbox from an EndFedz EF-QUAD (with Vibroplex’s blessing) so I could transmit at more than 25W. On all three bands I have very nice SWR - for 20m and 10m around 1.1. For 40m 1.1-1.5.

Today I switched over to using the radiator that actually came with the EF-QUAD matchbox - heavier gauge wire and full-size at 40m rather than being electrically-lengthened like the Mk2. And I seem to be getting better receive performance with it.

However, I can’t get SWR much below 1.5 on any of the bands, despite the charts that come with the antenna saying it should bottom out at 1.1. By trimming (well for now, folding back on itself) I can move the minimum SWR frequency but I have not been able to make SWR go lower.

Everything in the shack is the same. And the house end of the antenna is in the same place. Now, since this radiator is 25’ longer than the other one the far end is in a different place, making this radiator actually further off the ground than the other one. However it also goes closer to trees. It is not touching any branches or limbs but does come within a couple of inches of some decent-sized limbs in a few places.

Because of my living situation I'm pretty constrained as to where I can have the feedpoint of the antenna -- I was allowed to screw a hook into the exterior windowframe. The matchbox is is attached (with a short bit of rope) to that hook. A 1-foot length of coax connects to the matchbox and to a common-mode choke just inside the window. Then a 12-foot length of coax goes from the common-mode choke to the radio. This is all the same with both radiators. There is no ground or counterpoise because I'm not allowed to run a wire down from the window. And with both radiators I measured the SWR with an antenna analyzer by connecting it to the cable that connects to the radio -- because I wanted to measure the SWR the radio sees.

After getting nowhere with reducing SWR I decided to measure the SWR before the choke. When I did that, the minimum SWR dropped to 1.1-1.2 on most bands, and dropped noticeably on all four bands. With the previous radiator I had measured SWR both at the radio and before the choke and with that radiator they were virtually the same. So I assumed (danger will robinson) that would be true with this radiator. Obviously, apparently not.

Do I just live with it (my IC-7300 internal tuner can deal with it, though it does mean some power loss -- though with only 13 total feet of coax probably not too much)? Do I drop the choke? I've done some reading in a bunch of places and things seem to be all over the place. From what I've read it appears a common-mode choke is wise when you have a random wire end-fed. But for an end-fed dipole (like this) there seems to be much more conflict about whether a CMC is needed or not.

Update with measurements:

Here are some measurements I've made to try to figure out what's going on:

  1. Remove antenna from choke, attach dummy load to choke, connect antenna analyzer at radio end of the cable between choke and radio: dead flat at 1.0 on all bands.
  2. Attach antenna analyzer at choke end of the cable to that goes from antenna to choke (i.e. measuring close to feedpoint with no choke): SWR bottoms out at 1.2 in 40m band, is under 1.5 for the entire 40m band.
  3. Connect the antenna/choke cable directly to the choke/radio cable (i.e. no choke) and attach analyzer at radio end of choke/radio cable: SWR still bottoms out at 1.2 and is still under 1.5 in the 40m band, but the frequency of minimum SWR drops a little.
  4. Connect the antenna/choke and choke/radio cables to the choke, attach analyzer at radio end of choke/radio cable: SWR now bottoms out at 1.6 and gets over 2.0 at the ends of the 40m band. Minimum SWR occurs at the same place as in (3).

So the cable from radio to choke doesn't affect minimum SWR on its own (though it does change the frequency where the minimum SWR happens). And the choke doesn't affect minimum SWR on its own, but choke AND cable affect minimum SWR. But they didn't affect it for the other radiator.

So even if it isn't that big a deal to operate with the higher SWR I'm curious as to why this is all happening.

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    $\begingroup$ The product page is short on details, but "Electrically lengthened" and thin wire probably involved some loss. Now that this is gone, you're seeing higher SWR. But don't put a choke on the coax of an end fed... $\endgroup$
    – tomnexus
    Commented Sep 20, 2021 at 5:06
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    $\begingroup$ Why do you think that CMC on the outside of your coax shield is affecting the VSWR on the inside? $\endgroup$
    – Mike Waters
    Commented Sep 20, 2021 at 13:13
  • $\begingroup$ @tomnexus is right about not using a CMC. Unless you have a suitable wire(s) (that is, counterpoise or radial) connected to the antenna end of the shield, the outer part of the coax shield is actually part of the antenna and also radiates. $\endgroup$
    – Mike Waters
    Commented Sep 20, 2021 at 13:18
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    $\begingroup$ Remember with an end-fed, the coax, the rig, the microphone cord, the operator, the PSU and the mains connection are all part of the antenna. If you disconnect the radio and connect a (battery powered) antenna analyser then you've changed the antenna configuration. For most representative measurements, connect the analyser instead of the radio, but also connect the shell of the connector to the body of the radio. This doesn't matter so much with centre fed antennas because the coax isn't carring much current. $\endgroup$
    – tomnexus
    Commented Sep 20, 2021 at 16:34
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    $\begingroup$ "I was allowed to screw a hook into the exterior windowframe." "I'm not allowed to run a wire down from the window" What kind of a landlord allows their tenant to screw a hook into the window frame but not have a wire dangle on the side of the house? Crazy rules if you ask me. $\endgroup$
    – pgibbons
    Commented Sep 21, 2021 at 10:13

1 Answer 1

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A choked end-fed antenna is (or acts just like) a highly off-center fed dipole, where the length of any stub between the common mode choke and the unun/transformer/feedpoint sets the off center ratio and thus the impedance at the antenna feed point. Your current 1 foot stub is less than 1% of the antenna, whereas a few percent is usually recommended, that percentage depending on the transformer ratio of the unun. So I would experiment with different lengths of stub between the choke and the unun/balun. Be careful, since that stub will be at a high voltage node of the antenna, so calculate the radiation exposure for anyone potentially located near that stub.

With no counterpoise and no choke, the coax and the transmitter (and its ground, power, key, mic, and computer connection cables) often becomes the high voltage end of the antenna. Even with a choke, the feedline will be a conductive object very near the antenna's high voltage node, and thus could still pick up (and re-radiate) some RF, and thus cause pattern distortion.

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  • $\begingroup$ FWIW, it's a 7:1 transformer to create a 49:1 impedance transformation to take the 2500 ohms at the end of the antenna down to close to 50 ohms. I probably won't be able to leave extra feedline out there, but I can probably temporarily try it to see what effect it does have. It will be interesting to find out. $\endgroup$
    – QuantumMechanic
    Commented Sep 21, 2021 at 21:11
  • $\begingroup$ This web page: aa5tb.com/efha.html , recommends a counterpoise length of 0.05 lambda. The same length of feedline between the transformer and the choke should serve as a rough equivalent. Might work as well inside as outside the window as long as you keep it away from metal objects. Or if the window frame is metal, it might even work as the counterpose. $\endgroup$
    – hotpaw2
    Commented Sep 21, 2021 at 21:43

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