An alternative 2m antenna

After busting a gut to get my Diamond X30 up in the sky I was slightly disappointed with the absence of 2m activity. Admittedly I had a cracking WBFM reception for commercial radio and the POGSAC pager things I have been playing with utilising an RTL-SDR dongle were much improved, but I seemed to be missing a trick.

From consulting with the mine of information who is Mike (G0JMI) of Alton Antenna Arrays it transpires that as good as the Diamond X30 is, like everything it has its limitations.

Basically, the X30 is very similar to other verticals and its radiating section is about 3/4 wavelength on 2m (or there-abouts).

Three Qtr Wave Vertical Radiation Pattern 001

Above is the predicted vertical-plane radiation pattern for this form of antenna when used on 2m (145MHz) and you can see a bit of a gap at the lower bottom end where we would like energy to be if we are going to work stations at ground level.

When used on 70cm (434MHz), the vertical works as a 9 1/4 wavelength antenna and throws more radiations laterally making it better for working stations at ground level.  The predicted vertical-plane pattern for 70cm is below.

Nine Qtr Wave Vertical Radiation Pattern 002
The alternative 2m antenna to overcome this gap is a 2m half wave dipole.

Half Wave Vertical Dipole Radiation Pattern 003

As can be seen from the predicted vertical-plane radiation pattern for a 2m (145MHz) dipole and you can see that lots of useful power goes horizontally to allow us to work stations at ground level.  This antenna on 70cm has a similar radiation pattern to the 9 1/4 wavelength type outlined above, so is good for that too.

The X30 as a 2m 3/4 wave vertical is good for working stations through satellites because of the amount RF thrown upwards.

As such, making a simple half wave dipole for 2m work, especially joining in the local club nets seemed like the ideal solution.

The components to build one were very few and far between – a dipole centre, two lengths of aluminium tube, some RG58 coax and half an hour.

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And there you have it! The hardest part was winding the 6 turns into the coax to make the choke-balun and getting it to stay there. If you’ve ever tried wresting an uncooperative snake you’ll know what I’m talking about!

Tuning the dipole down to centre frequency involved nibbling off 3mm lengths from the dipole arms until the SWR was down to 1.2:1. That proved a little problematical as I was relying on the FTL meter attached to my rig and when I got down to the predicted length all of a sudden the SWR which had been coming down nicely started to increase!

With a bit of assistance in the form of someone more knowledgeable, an antenna analyser and a trusty Oskerblock SWR200 dating from the 1970’s, all was eventually overcome. If anything it demonstrates that reliance on cutting edge tech to make things isn’t necessarily the way to go. The MFJ-269C antenna analyser wasn’t as accurate or reliable as we’d hoped and by relying on gut instinct and the two analogue meters of the Oskerblock we got there.

I’ve now got a very nice homebrew antenna that is great on both 2m and 70cm’s and can be used for both the local club net and satellite work!

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