Hamstick Dipole: Why is SWR changing when I move the coax?

[k2mm]

Hello,

I have been going crazy trying to tune my 20m hamstick antenna. It is mounted 3 feet away from my balcony and about 30 feet AGL. The antenna is not grounded, but shouldn't require it since this is a dipole. I used an antenna analyzer to adjust the antenna at 14.175MHz with a reasonable SWR. Everything looks great, so I moved the coax with the analyzer about 15 feet into my apartment and run the analyzer. The antenna analyzer no longer reports anything close to what was tuned when I was on the balcony. I am using RG8X coax, so I cannot imagine an interference issue. I have two loops of the coax secured below the feedpoint to the dipole.

I would really appreciate your suggestions as to what is wrong and how to fix it.

Thank you for your interest and time,

Mike

 

[prcguy]

You probably have a lot of common mode RF currents flowing on the coax shield and I suspect a good ferrite loaded choke balun will calm it down. You will have some VSWR because two short loaded hamsticks in a dipole config will not give a great match to 50 ohm coax, so the coax shield has become a radiating part of the antenna. You need to decouple the coax from the antenna and a couple of turns of coax will do nothing for the problem, you need a real effective choke balun.

Once you get a good ferrite choke right at the feedpoint you should be able to touch the coax or move it around with little to no change in VSWR.

 

[k2mm]

Thank you very much, prcguy! I will get one and report the results. You rock!

 

[Ubbe]

I've looked at some YouTube videos of hamsticks and all of them show people use the balanced dipole directly connected to an unbalanced coax. There's a reason to why there are baluns you can buy and use in these situations. The crude solution are a choke balun to at least get rid of the HF on the outside of the coax and could be a good solution if you work an antenna over several frequency bands and at high transmit powers, but a proper galvanic isolator in a transformer balun design are probably the best way to interface a dipole to a coax to not get distortion of the radiation pattern.

But that hamstick seems to most often be used with a grounded mounting piece that shortcircuit all signals from one of the elements and will then only work as a counterpoise, a GP antenna with only one ground plane element, and not as a full dipole that adds the signal from both elements. If that mount are used it needs to be properly grounded as the antenna doesn't operate as a dipole. Even when using a choke balun, a free floating ground on the antenna will reduce the signal levels to and from the antenna as the whole antenna will "move" RF wise with the radio wave.

/Ubbe