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N2VHZ

Joined: Sun, Apr 22nd 2012, 18:58 Roles: N/A Moderates: N/A

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adjusting SWR and hamsticks - help?? KM6SQB on 3/11/18
Quote by N2VHZ


I am just getting back in to ham radio after a 20 year absence.

I needed a quick, non-wire portable HF antenna for 20M -10M (the staff at the local park doesn't like
the idea of a support rope attached to their trees). I decided to go with some MFJ hamstick style radiators ,the MFJ-347 dipole mount and a 15ft fiberglass pole I had on hand.

I know the 15ft pole is rather short, but I got the 10M and 15M sticks to tune up nicely. I did have to offset the lengths of one stinger by 1/4"-3/8" compared to the other side to get the best SWR (around 1.2:1) on both bands.

On 20M I can't get anything bellow 2:1 for SWR. I checked the sticks for continuity between the base threads and stingers. I pulled the stingers and checked for the resonant point of the coils alone which came out around 27.3 Mhz with a 1.3:1 SWR. I tried several different lengths and types of coax with no improvement.

I then tried offsetting the length of 1 stinger- I got the SWR down to 1.15:1 but about 7" of one stinger was down inside the coil which isn't allowed. Obviously something is unbalancing the antenna. Is the 15' height just too low for 20M or perhaps the antenna is coupling to the coax?

Thanks,

Mark
KE8DIH


adjusting SWR and hamsticks - help?? KM6SQB on 3/11/18
Quote by N2VHZ

Did you try mounting the dipole in a vertical orientation? This would help with your height above ground situation and lower your takeoff angle. This gentleman made it work quite well:

https://youtu.be/CbQwLPVmG70
adjusting SWR and hamsticks - help?? KM6SQB on 3/11/18
Quote by N2VHZ


If the coax runs straight down from the feedpoint to the ground, it shouldn't be coupling much to the antenna.

Possibly something's wrong with one of the antennas; however at 15' above ground on 20m the ground influence is pretty powerful and you may never achieve a good match to 50 Ohms. That would occur at about 32-33 feet above ground.

A VSWR of 2.0 isn't bad, I'd have no problem dealing with that and just using it.



adjusting SWR and hamsticks - help?? KM6SQB on 3/11/18
Quote by N2VHZ


If you plan on working 15m and 10m on that antenna too, I'd set it back to normal and pick up a decent tuner to work 20m with. If you place the tuner right at the feedpoint of the antenna (which you totally can do in your scenario) there will be negligible loss and you might even be able to get 40m, 17m, and 12m out of it too.

I used the MFJ hamstick dipole years ago and found the same problem with it; at 1.4wave up you have to set one stinger in much shorter. If you decide to keep it that way (or anyone else reading this) be sure to clip the extra length from the stinger. If you don't, the loading coil will pump power into the stinger inductively and eddy currents will eventually destroy the antenna. I found this out the hard way.


adjusting SWR and hamsticks - help?? KM6SQB on 3/11/18
Quote by N2VHZ


I am just getting back in to ham radio after a 20 year absence.

I needed a quick, non-wire portable HF antenna for 20M -10M (the staff at the local park doesn't like
the idea of a support rope attached to their trees). I decided to go with some MFJ hamstick style radiators ,the MFJ-347 dipole mount and a 15ft fiberglass pole I had on hand.

I know the 15ft pole is rather short, but I got the 10M and 15M sticks to tune up nicely. I did have to offset the lengths of one stinger by 1/4"-3/8" compared to the other side to get the best SWR (around 1.2:1) on both bands.

On 20M I can't get anything bellow 2:1 for SWR. I checked the sticks for continuity between the base threads and stingers. I pulled the stingers and checked for the resonant point of the coils alone which came out around 27.3 Mhz with a 1.3:1 SWR. I tried several different lengths and types of coax with no improvement.

I then tried offsetting the length of 1 stinger- I got the SWR down to 1.15:1 but about 7" of one stinger was down inside the coil which isn't allowed. Obviously something is unbalancing the antenna. Is the 15' height just too low for 20M or perhaps the antenna is coupling to the coax?

Thanks,

Mark
KE8DIH


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