Questions regarding a "simple" HF half wave dipole and RF on the feedline braid. - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 01, 2020, 10:41:36 am
content: I have worked digital LF (136kHz) for so long I am only now, during the virus lock down, returning to talking to real people and putting up HF aerialsI have put up a 20m half wave dipole and it seemed to work ok, but on 50 Watts it would kill my Kenwood TS-590S computer access software on my lap top near the transceiver. My AIM4170 showed resonance at the same point as a below 1.4 to 1 SWR minimum, and the rig's SWR meter showed under 1.5, too. It became annoying so I built an RF current meter that uses a clamp on ferrite as part of the transformer. It was a kit by the late G4HUP that I have had knocking around unbuilt for years.http://huprf.com/huprf/rf-current-meter/It has two ranges, 0 to 100mA range, and o to 1 Amp range. I calibrated it and on the 100mA range it takes over 5W to go FSD. I was concerned to find it banging in to the end stop when I transmitted a 50W CW carrier into the antenna, meaning well over 5W was on the screen of the co-ax feeder near the rig.I am using a commercial 1 to 1 balun at the centre of the dipole, rated 1.5kW over a range of 1.8 to 30 MHz. It is in a sealed plastic pipe and I have never been 100% about it as the originator's web site never said, nor showed what was inside it... (EDIT) I managed to get inside it without destroying it, I have questions and photos, no ferrite beads or toroids, but they are for a different thread, and later).So I made my own 14 metre dedicated common mode choke, winding 11 turns of RG-58 co-ax on to an FT240-52 toroid, as per the details on the late G3TXQ's web site athttp://www.karinya.net/g3txq/chokes/I replaced the commercial "balun" (and it this point express frustration that newbies like myself have to try and decipher if even commercial manufacturers mean a balun or a choke in their blurb, they seem to be used at random for different or the same things), with my home brew one. It's in the same place directly between the dipole's legs, and fed with the same RG-58 of about 50 feet in length direct to my TS-590S antenna socket.This choke improved current on the co-ax shield to only about 2/3 deflection on the 100mA scale, so well under 5W on a 50W carrier. For some reason I decided to utilise the good advice someone on this forum gave me, to put the hanger ropes on pulleys, and to try shifting the dipole along, horizontally. One end is getting into the edge of woodland, the other leg is near (within 15 feet) of my lattice work 40 foot wind up mast. I am glad I did have this idea as it was immediately apparent even movements of 15 feet, horizontally, made a big difference. Further towards to the edge of the woods made things very quickly worse. So did bring one leg nearer the steel mast. I experimented further and found a "sweet spot" where no RF current meter deflection occurred at all.So what do I make of all this? Are objects UN balancing the operation of the dipole so one leg is "working harder" than the other?My questions are, is this due to the dipole being near trees and / or the steel mast? Should I worry? I think I must if it also puts a lot of RF on the feedline braid.Finally, and in some ways mainly, I am still far from clear as to whether a half wave simple dipole should have just a simple choke (a toroid of type X wound with Y turns of co-ax, the feeder co-ax to one end of the single winding, the legs of the dipole to the other end)? Or do I need instead, or as WELL, a true BALUN, in my mind a toroid with more than one winding, it's main purpose being mating an un-balanced coaxial feedline to what is a hopefully balanced half wave dipole antenna?Thanks for reading! Any advice very welcome, thank you.

author: christopher iles
author_rank: Regular Contributor
timestamp: May 21, 2020, 08:38:42 am
content: Hello Chris,For years I have used a simple choke balun which is 7 or 8 turns of RG58 coax about 7 inches diameter (I use a paint tin as a guide).The air wound choke is always sited as close to the feed point of the dipole as possible and I have never had any common mode rf problems even running the legal limit on hf.It works for me, easy to try and very cheap. You will need between 18 and 21 feet of co-ax. Let me know how you get on.Kind regards,Chris, G0VOE.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 21, 2020, 01:59:41 pm
content: Thanks for that, I will try a coil of the co-ax as a choke. The odd thing about this 20 metre dipole is on RX it's very noisy. Another dipole cut for 40 metres that runs at an angle of about 45 degrees to it is very quiet and hears about the same. I also have a big horizontal loop of about 560 feet in circumference fed with window line at one corner, to a ferrite ring choke at ground level, for 160 metres. This is ULTRA quiet and by far the best for 20 metre reception.The whooshing noise on the 20 metre dipole is remarkably noticeable in comparison to the other two, and similar to an active wire loop Wellbrook antenna for LF to HF, which is deaf on 20 metres but excellent on MF and LF...

author: donmr
author_rank: Regular Contributor
timestamp: May 21, 2020, 11:04:44 pm
content: Is the coax shield grounded where it enters the house, and at the radio?Even with a perfect balun the shield is a wire hanging out there in a high RF field. Thus it can pick up your signal (as well as any others floating around) and bring it back into you shack. Good grounding is the first step in getting rid of this, ferittes are another thing you might try after grounding.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 22, 2020, 08:12:44 pm
content: No, to be honest the only ground is via the mains power feed earth pin to the transceiver. My equipment is in an upstairs room, so an RF ground would be about 30 feet long to the nearest practical earth rod I could install. I believed that would NOT be a good idea? I can try temporarily moving the transceiver downstairs near a decent earth rod so it would have only a few feet to it if you think I would see a fix by this, but long term it has to stay upstairs.

author: M0HZH
author_rank: Regular Contributor
timestamp: May 22, 2020, 09:47:25 pm
content: When the coax is coming back to the building, is it sitting on the ground parallel / under the antenna ?There have been made many advances in choke BalUn designs in the last 2 decades, due to better materials becoming available and the required test equipment becoming more accessible to amateurs. Achieving high choking impedance, wide bandwidth and good power capability at the same time is no trivial matter and most designs are either just based on a flawed concept (like the air-wound "choke") or simply do not achieve sufficient choking or they're too narrowband. This document will answer most questions on the matter, if you have time to go through it:http://audiosystemsgroup.com/CoaxChokesPPT.pdf(TL;DR: 12 to 14 turns of RG-400 on FT240-31)As for grounding, for SAFETY reasons you are supposed to ground the coaxial right under the antenna and also right at the point of entrance in the building; this point of entrance grounding should be bonded to the electrical service grounding on the outside of the building. If this is done correctly, the safety grounding will also ground most stray RF circulating on the outside of the coaxial (either because it made it past the RF choke or because the coax picked up some RF being in the antenna near field).Worst case, you add a second choke on the cable (preferably closer to the transmitter than the antenna) but if that's needed there are certainly bigger problems that need adresssing.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 25, 2020, 11:25:59 pm
content: The feedline co-ax has a balun for 20 metres at the centre of the dipole based on the charts at the late G3TXQ's site. The feeder co-ax comes off at near 90 degrees then is brought in to the shack in a lazy curve The feedline is about 50 feet in length overall. It does not run close to parallel with the dipole legs. But it does not reach down to the ground at any point.I will modify things to lengthen and ground the shield, and if that fails, try a shack end balun, too. may thanks for the detailed reply.

author: bob91343
author_rank: Super Contributor
timestamp: May 26, 2020, 12:12:32 am
content: I have used coaxial chokes on all my antennas with great success. I don't like commercial baluns because they might contain ferrite cores whose linearity is in question and may generate harmonics.I run a full kilowatt and get great reports from all over the world.

author: tkamiya
author_rank: Super Contributor
timestamp: May 26, 2020, 12:24:29 am
content: How is your PC connected to the Kenwood? Is this part well shielded?Being near something changes effective length of the antenna. Thus it could well be unbalanced. I typically don't bother with baluns. In practical use, it never seem to make any difference. If it's low to the ground, how is your coax routed? Perpendicularly or near parallel? Even with a balun, that could cause RF pickup too.

author: M0HZH
author_rank: Regular Contributor
timestamp: May 26, 2020, 08:42:17 am
content: Quote from: bob91343 on May 26, 2020, 12:12:32 amIn a current balun, the ferrite core will only "see" the common mode currents - the RF power that returns on the outside of the coaxial cable. It doesn't really matter what power you're running through it. Plus, a single FT-240-31 core can handle 1kW of power or more, probably has more volume than the ferrites in your amplifier's output transformer.The coaxial choke / air core balun / "ugly balun" is not really a BalUn in the sense people understand HF choke baluns. It's really an inductor, doesn't achieve high choking impedance, is very narrowband and mostly reactive (instead of mostly resistive like a proper ferrite core choke balun). It may work reasonably well at one specific frequency (where the inductor self-resonates: that's why it's used in some VHF/UHF antennas), but at HF over 4 octaves it's just all over the place. If your system doesn't really need a choke balun it wouldn't make any difference, when very little choking is needed it might be just enough to push things over the edge, but in an antenna system where proper choking is needed it will only waste time.

author: donmr
author_rank: Regular Contributor
timestamp: May 26, 2020, 02:18:14 pm
content: A long ground wire is not as good as a short one, but it is still better than no ground.

author: cdev
author_rank: Super Contributor
timestamp: May 27, 2020, 03:05:03 am
content: Maybe you need some means of galvanic isolation?Quote from: Chris Wilson on May 22, 2020, 08:12:44 pm

author: iMo
author_rank: Super Contributor
timestamp: May 27, 2020, 11:53:56 am
content: Quote from: Chris Wilson on May 01, 2020, 10:41:36 amA common mode choke wound on a toroid requires a careful construction - you have to use a special winding style such you minimize the capacitive coupling inside the choke - ie. you have to wind 5 turns and then go to the opposite side of the core and wind the remaining 5 turns.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 07, 2020, 10:26:49 pm
content: Thanks for the replies and apologies for my late return, for some reason my software failed to indicate I had received any responses.