Tuning an Ampro 20m vertical antenna

1

Been using this vertical as a mobile user with a magmount on my car. Now experimenting with the same antenna as vertical in my garden with 2X16.5 feet radials attached with a crocodile clip on the outer coax braid. Will tune only to 12.980 with swr 1.2. To get into tune at target of 14.130 swr calculator shows that I need to reduce whip by 43.73cm ( a large amount) I did this and swr off the scale and no matter how much I adjust cannot get this antenna into tune on 20m band. No short in coax feed and everything else seems fine so a bit of a puzzle. Back to use on the car until I can crack this. Any ideas/solutions will be gratefully received.

Thanks

Doug MM7DSA

2

I wonder if it’s just the radials - two is probably not enough and they are a bit long - what is the ground? concrete or soil/earth? Why not wait for a wet day, and put the antenna on the ground, and try it there?

3

Thanks Paul. Ground is soil under grass. Will try with shorter and perhaps more radials. Already soaking with radials about 2mtrs above ground. Shorter and laying on ground to try next. Softly softly a bit at a time may get this sorted. Using a dirty balun with 5 turns coax around a 5" plastic pipe just below antenna. Will also try with more turns and no Balun to compare results.
Thanks and will keep reporting as I go.

4

I’d bet the balun is the culprit - the antenna is unbalanced, the feeder is unbalanced and the radio is unbalanced - so nothing balanced needs unbalancing - and I bet if you run the antenna, which is supposed to be 50Ohms anyway, direct to the radio it will work better.

5

Thanks for that. Just unwound balun but swr went up slightly. Shortening radials work but needs more shortening, leaving until daylight tomorrow to experiment further.73 Doug

6

This dirty Balun? Is it one of the simple ‘air’ baluns that aren’t baluns? I’ve never been convinced on them. If that’s what you had, they’re meant to remove the unwanted RF current on the cable screen, and usually there shouldn’t be any!

I wonder if maybe it’s always been a bit long? The choke might have messed up your original tuning?