DIY Pole

Rather than go to a whole load of work and expense, I have looked at ways to use mass market domestic antennas to build custom receive, amateur transceive and broadcast antennas.

We've had a lot of fun with the Maxview dipoles from B&Q. This was a kit which had everything you need for a Band II dipole. It turned out that it matched quite well for TX with the inclusion of a ferrite bead to prevent currents flowing along the outside of the coax. It was great because you could chop it down to go up band and insert stock 10mm tube to do stuff like Band I link antennas.
B&Q have now discontinued the Maxview kit but instead, they do a single element folded dipole kit from www.masterplug.com which looks very promising, more than I can say for their website.
For a start, at the feedpoint, there's a balun. It's probably only good for 8W or so but there's probably enough room to include a higher rated balun which can deal with 50W. Rather than being the usual hard wired crap you see on conventional domestic fare, it feeds out to an 'F' socket includes a boot. It looks very easy to change the 'F' for a BNC.

Moving on from the Maxview FM dipole which is no longer produced, I've been looking towards other products to modify. One which currently appears on B&Q's shelves for £12.98 is the Masterplug Omnidirectional FM Dipole which is manufactured in Italy under ISO 9001.
This unit is a folded dipole mounted on a short 18mm square boom . In its original form, it appears quite suitable for use with low power <8W band II transmitters but given the tiny balun it would not be able to withstand any substantial power. I originally found it to have <0.5W reflected to 5W forward at 87.7MHz with a wooden support structure.
The original Ruthroff 4:1 balun has a core of 7x5.8x4.2mm and uses EC wire of approximately 0.3mm overall. A Ruthroff balun is a crude design and is essentially an autotransformer. It relies on having a power splitter and just one single 1:1 transformer to achieve an overall 4:1 impedance transformation. We shouldn't knock Masterplug for using this simple design. I've seen may domestic folded dipoles which don't even have a balun at all.
It would of course be possible to make up some sort of co-axial balun but this would not fit in the feedpoint housing and is by nature tuned so would not be suitable for experimenting between different bands.

With my first prototype, I used SWG 24 EC wire on a larger binocular core. I did not use the Ruthroff design, fine as it may be for a receive antenna, it is too crude for a transmit antenna, especially at high power under less than ideal conditions.

I took my inspiration from this excellent article by Chris Trask

The various tappings

It looks scary but not venomous

The balun installed into the feedpoint housing

The improved balun shown in figure 4 in the PDF is essentially a power splitter feeding the primary windings of two 1:1 transformers. The secondaries of these transformers are coupled in series in order to drive the required 300Ω load. It's entirely symmetrical and both transformers can be wound on the same core, thus ensuring that incommensuration in respect of permeability would not be an issue.

I managed to get four turns around this core with SWG 24. Four lengths of 280mm are used.

I also replaced the original 'F' connector on the feedpoint housing with a 50Ω chassis mount BNC female. The steel support bracket has a cut out which includes a flat to prevent the connector turning and the BNC can be added with no drilling or filing. One slight disadvantage here is that the BNC protrudes much further into the housing, thus wiping out a lot of space. I still found that the transformer could sit to one side without any problems. Of course an 'F' could be used, this, coupled with Sat co-ax may be the preferred route for some users.

And indeed, a 4:1 impedance matching transformer will mathematically only be working correctly with a 75Ω feed.

Note that by substituting the standard 'F' with a BNC, the tightening nut moves from being on the outside of the plastic housing to the inside.

The original kit is supplied with a rubber boot which happily accommodates an 'F' connector, I will have to investigate as to how well it accommodates the BNC.

There is a strange pressing within the steel socket mounting bracket which appears to serve no purpose. This could be easily drilled out to allow more space for the balun.
This I did with the second prototype. It left a useful amount of extra space.

The hex bolts which feed through to the elements would possibly benefit from being replaced with stainless steel fixings. Dimensions for this fixing are M4x10 ISO. I couldn't get hold of 10mm fixings so I used 12mm instead with a plain and a crinkle washer on each terminal so that the end of the bolt didn't protrude too far into the hex socket cap nut.

Power testing was carried out but unfortunately it did not like up to my hopes. VSWR remained low but at 50W, the balun heated up very rapidly. I can only assume that this antenna would only manage a maximum of 10-20W continuously rated.

The next thing is to get the whole thing to telescope so that it can be tuned to different bands. My target is to go down to 4m, and if mechanical stresses permit, 6m. On another antenna, I want to take it up band to airband, 2m Marine VHF and even UHF. It would be fun to make a four stack array based on these things.The inside of the trombone arms is 10mm and the outside is 12mm. I've got a useable straight section of about 560mm so it would seem easy to make an antenna which will telescope between 6m and 4m although I'm not sure if I can make the same antenna collapse back to band II with the same inner rods. It seems sensible to anchor the 10mm right into the centre point so they can take advantage of the through-bolt and have maximum rigidity. This will also lessen the number of hose clips needed by two.

The supplied boom also looks very short even for Band II, it looks less than 1/8 lambda. I guess this is ok in horizontal polarisation configuration, or bolted to a gable but could cause problems at a lower wavelength. The boom is 18mm square section with 16mm internal square. Maybe I can get hold of something slightly thicker than 1mm wall thickness to beef it up a bit. B&Q don't seem to carry this stock currently.

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