2
$\begingroup$

I use a hex beam to operate from 20 meters to 6 meters with amazing results.

I wish to operate 40 meters and 80 meters also, and can only really install an end fed long wire due to my installation location.

Recently I installed an end fed long wire which comprises of a 9:1 unun with 25.6 meters ( 84 foot ) of wire. This is installed up high in a straight line. I don't have a counterpoise connected to the unun, and my coax cable feeder length from shack to antenna is 20 meters long.

The result is that the SWR is good on 40 meters, but extremely noisy on receive compared to the hex beam. So I use the hex beam for receive on 40 meters instead.

For 80 meters, my internal ATU can not even tune for this band which i'm surprised about.

Onto the main problem. As soon as I transmit on the end fed, it seems that RF is getting back into the shack, because the serial port on my computer locks up and I can no longer control the radio via the computer. This never happens when I transmit on the hex beam.

I have a 1:1 choke balun where the coax enters my shack but it still doesn't seem to solve the issue. I was advised not to put the choke at the antenna input because the coax must be used as the counterpoise. Others suggested to use a 49:1 balun instead of 9:1 balun, but this is not meant to be a perfect end fed half wave, more of a random length end fed.

Any experience with something similar or suggestions on how to improve this would be appreciated.

I was expecting an end fed with a length of 84 foot so high from the ground to perform incredibly well, but unfortunately it does not.

$\endgroup$
5
  • 2
    $\begingroup$ There's really no way you could take that same length of wire and, say, make some kind of doublet or OCFD out of it? Maybe even shorten it up slightly so that the ends are at least a few feet from any structures. And then have a balun at the feedpoint as well as a good choke at the near end. Or in general: please share a little more info about your setup so that we can suggest things you may not have thought of. $\endgroup$ Apr 7 at 15:52
  • 1
    $\begingroup$ Do you have chokes on all the cables from the mast (rotator, beam etc), or just the end-fed one? And what does your bench look like - computers, radios, serial ports, mains power supplies, coax from outside and earths? $\endgroup$
    – tomnexus
    Apr 7 at 16:12
  • $\begingroup$ @hobbs-KC2G My antennas are installed on a building rooftop. It would be a bit tricky to get a center fed dipole or doublet there due to the location of my feedpoint. My feedpoint is on the very left side of the building (under my hex beam) and it therefore gives me the rest of the rooftop to the right to play with for the actual antenna wire, so it really favors an end fed. $\endgroup$ Apr 7 at 21:11
  • $\begingroup$ @tomnexus I don't have chokes on all the cables. Only the choke at the feed point to the hex beam which is part of the antenna, and then a choke on the coax feeder where it enters the shack for the end fed (but not for the hex beam) $\endgroup$ Apr 7 at 21:13
  • $\begingroup$ @Engineer999 there's a reasonable OCFD design with the feedpoint 20% from one end and a 4:1 balun. I think you could very easily do that for 40m (68 feet long). 80 is a harder nut to crack... but what about a horizontal loop? $\endgroup$ Apr 9 at 18:28

2 Answers 2

1
$\begingroup$

The higher receive noise is possibly an indication that RFI from your house and/or computer is getting up to the feed point of your off-center fed dipole (what you are calling an "end fed") via your feedline. Thus RF from the antenna might be using that same path to get from your antenna into your computer USB cables.

The coax doesn't have to serve as the (main) counterpoise. You can instead put a common mode choke at the feed point to the antenna, and add a wire angled away from the coax as a better counterpoise. That will add more separation between the high voltage node at the very ends of your antenna from capacitive coupling to your coax feedline. You may have to tune the length of the counterpoise to best match your frequencies of interest. e.g. find a counterpoise length and UNUN ratio that best lowers the SWR without a tuner, and then use your antenna tuner to lower the SWR even further.

In addition you may also need coax chokes on your feed line at the shack entry point and at the transmitter to reduce RF through your USB cables. The spacing (length of feed line) between the chokes can make a difference. Chokes on both ends of your USB cables may help as well. If the grounding of your transmitter is both fatter and shorter than your USB cables, that may also help reduce the RF going thru your USB cables.

$\endgroup$
1
  • $\begingroup$ Thanks for your reply. This is what I was thinking to do.. put a choke at the coax feedpoint and use separate wire for a counterpoise. I am mainly interested to operate at 40 meters, so which length roughly would suit the counterpoise? Regarding transmitter grounding, my transceiver is not actually grounded. I am on the 10th floor of an apartment building. $\endgroup$ Apr 7 at 21:21
1
$\begingroup$

I'm no expert on EFHWs but here's what i've learnt with mine (happy to be corrected if i've got anything wrong by the experts of the forum).

Fundamentally, an EFHW (with no counterpoise, or connected to ground) is a wire connected to the centre conductor of the unbalanced coax only.

As there's no direct connection to the shield, and no other route, the main way for return currents to get back to the source is to couple and travel down the outer shield of the coax, and being on the outer-shield it of course makes it radiate and pick up noise.

A ground connection helps (for more than 1 reason) or a short counterpoise of around 0.05 wavelength long at the lowest band to add a bit of wire on the shield side (which makes it an OCF dipole technically).

As currents are travelling down the outer shield, then as hotpaw2 suggested a choke is needed near the feedpoint to prevent the CMC travelling down the outer shield and coming back that way. Even with a counterpoise or ground, a choke is a good idea. And a choke at the radio end is not a bad idea either to filter out any CM noise picked up along the way (prevention is always better than cure).

Since you're in an apartment, sounds like a 0.05L counterpoise is a good idea, as well as chokes. For a 40M fundamental EFHW, 0.05 x 40 = 2M long will be a good length. Connect it to the shield side and run below the antenna.

HWEFs are good, but they are notorious for CMCs for the above reasons (but can be tamed). I've learnt there's more than i currently know to eliminate (most) CMC and have low-noise pickup such as that used in the commercial Hyendfed antennas and others, but that's beyond my knowledge currently :)

Anyway you have a few easy things you can try to hopefully improve your setup.

Cheers!

$\endgroup$
6
  • $\begingroup$ I read about similar CMC issues when i was building my EFHW. To be clear i didn't build a long wire/random wire but an EFHW for 40m-10 with a coil for 80m. I based my design on the following - youtube.com/watch?v=RsgdcHeP6S0 I wound a 49:1 based on the design from the following website - hfkits.com/… The EFHW is my main antenna (due to the limited space i have) and is setup as an Inverted V with the middle at about 33ft high. I use 3 240x43 cores, along with a ground and have it grounded again using an earth ro $\endgroup$ Apr 21 at 23:36
  • 1
    $\begingroup$ Welcome to HAM.SE. Make sure you take the tour if you haven't already. Most importantly, SE sites are not threaded forums, but rather are Q&A sites. I'm not sure how this answer pertains the the question asked. Rather, it seems to be more of a "me too" comment. $\endgroup$
    – user21417
    Apr 22 at 0:12
  • $\begingroup$ It was meant to share an experience, provide alternate options, please feel free to delete the post mate. $\endgroup$ Apr 23 at 1:56
  • $\begingroup$ @JohnVE3WNA is correct, and taking the tour is a great suggestion. However, since you're new to this site I'm not going to delete your answer. (Moved to comment on this page.) Welcome to ham.stackexchange.com! :-) $\endgroup$
    – Mike Waters
    Apr 29 at 21:11
  • 1
    $\begingroup$ Also, this would be allowed in Ham Chat. Once you've gained some rep, you'll be able to post there. $\endgroup$
    – Mike Waters
    Apr 29 at 21:13

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .