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looking for a mentor in antenna building

PostPosted: Sat Oct 10, 2015 4:46 pm
by 2E0DED
due to the fact i have stopped going to my local radio club i am in need of a antenna building mentor

i am trying to build with complete lack of success a linked dipole and want to ask where i am going wrong

Re: looking for a mentor in antenna building

PostPosted: Sat Oct 10, 2015 5:52 pm
by G4NVB
What problems are you having? Have you tried making a single band dipole and then progressing from there?

Have you seen:-
http://www.sotabeams.co.uk/linked-dipoles/

Re: looking for a mentor in antenna building

PostPosted: Sun Oct 11, 2015 11:30 am
by 2E0DED
So what am I doing wrong. I am trying to build a simple pre tuned linked dipole for 50w or less use for SOTA reasons. So I want to preset the lengths so I don’t have to carry an ATU and can simply set up and run with the 817 I have.
I have to hand a wire cable of 13/0.1 white bell wire. I am following the standard SOTA design and spec and building without a balan as the first attempt with balan was awful.
I am on the first set up which is for the 10m band and should be in tune around 28,150 kHz at 2430mm long.
As I planned to trim to tune I have started at 2430mm and expect to trim back to allow for slack cables etc else where.
It is supported on a nylon washing line with plastic pegs as a support until finished. The feeder is 3m of rg58 fed alone the ground from the antenna tester, a AW07A from Feature Tech. this is sat on the work bench in the garden. Garden is fairly free of any visible steel or alli to distort the figures.

Readings logged as we went along.
Started at 21,845khz, 50 Ohm, swr 1;7
Tuned at 24,680khz, 48 ohm, swr 1;0

Chopped 200mm off the length by back bending the cable to allow for any adjustments.

So at 2230mm long we read readings off
28,100kHz, 50 ohm, SWR 5.5
tuned at 25,528kHz at 50ohm, SWR at 1.1

so we chopped another 200mm back, well back bent the wires to suit 2030mm long.So I am 400mm shorter than expected and in my eyes well past the sensible length of 2430mm ish.

So we tried again with another 200mm back bent so we are 600mm over short to me. And at 1800mm per leg try read the following readings.
28,150kHz, 75ohms, SWR 2.7
tuned at 26,160kHz at 50 ohm and SWR of 1.0
now the issue I have now is instead of building a quarter wave dipole of 2500mm per leg and a over total of 5000mm a standard half wave size I am now shooting down to a one eight wave and have missed totally the main target length

so where have I gone wrong , what have I missed out or have missed totally off or did not understand and why is at the usual figures given give or take a foot either way am I not getting a good resonance for the 10m band I require.
Also how do I increase the capacitance as well as some readings of low SWR also come with 30 ohm and 40 ohm readings and not the 50 ohm I wanted and when I tune up to 50 ohm we have a SWR of 2.2 and higher.
Whats sort of SWR does a standard linked dipole give off, I am aiming at 50 ohm and 1.5 or less, is that ok or should I be aiming at a different set of results??