A PORTABLE  VERTICAL  0.5 l

One day I had a phone call from old friend asking what sort of antenna he could put up in his garden. I knew he had a long wire as I was over some time ago to put it up. He is interested in DX also ( he comes over every year to my QTH for a week to use my radio equipment).  He has a very small garden and has problems with the owner regarding what he can put up. We talked about putting  up a vertical, he thought I meant a ground plain with 0.25 l long radials.   I don’t think I could get permission  to put the antenna up.   What if you had a

 0.5 l vertical which  has some small radials? he said, as we could get away with that, but how long are the small radials?  I do not really know but the only way is to build one and try it out. We decided to make the aerial either on 17 or 20 meters. I happened to have some scrap aluminium from old aerial projects  and CB Sigma half wave in a box unused. From experience in other projects when it is fully extended the length is about 17 feet. To calculate the half wave length in feet I calculated using the formula out of the ARRL Handbook l/2=468/ frequency, where the frequency is expressed in Mhz.l/2  is in feet.  For 20 meters the length is  (32’  11 “) we used a centre frequency of 14.2Mhz .  For 17 meters we used a centre frequency of 18.110 Mhz  the length is  ( 25’ 10” ).  I found a piece of 2 inch aluminium pole 12 feet in length which would be suitable for the base of the antenna.   With the sigma antenna and the aluminium pole this would give  a total length of 29 feet. In the local B& Q I found some small bore aluminium tubing so that I could extend the sigma aerial length. All the required aluminium tubing was  now at hand to fabricate either a 17 or 20 meter antenna.

However there were still two small problems.

1: To bring it to England I needed to bring it through the Airport Security and would be quite bulky to carry around.

2: The other way would be to take it as a portable to fit into a car boot

I needed to cut the pole into two parts and use an aluminium insert to join them together with bolts. The other problem I saw  was to join the two inch pole to the sigma antenna. For a reasonable price I had these two items made  Fig 1 is an insert to join the two pieces of two inch aluminium  pole together. Fig 2 is a tapered piece of aluminium with an inch hole drilled down the centre.This is to join the two inch pole to the sigma antenna. From the theory I know that the half wave which is end fed has a high impedance and I need to match this impedance to 50 Ohms  to give a good SWR and work efficiently. I mounted the antenna on a fence post with a couple of small radials at the bottom using a MFJ HF/VHF SWR ANALZER.  I discovered that the normal base impedance was between  400- 600 Ohms. I also found in the scrap box two balun one was marked 4-1 the other 6-1.   I connected a piece of coax to the HF Analzer and connected up the 4-1 balun ( one wire went to the vertical the other to the radials)  Tuning the Analzer  from about  13 – 19 Mhz I found a sudden dip and the SWR went to 1.2 – 1 which looked very promising.   But found the size of small radials effect the resonaze of  antenna so did by change size of verital rod I mount the verical on a wood post use to find out the effect of change rod and radials see fig 3

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I produce a table  show the effect of change the verical size and radials size  I set resonaze frequence to 18.110 Mhz