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March 2023 - A VHF/UHF Vertical Antenna
Before I moved home just over a year ago I had antennas for 2 metres and 70 cms. There wasn’t much activity - but it was nice to have. The old “white stick” tri-band vertical fell apart when I tried to remove it from the house brackets. Well, it had been working well for 20 years so it didn’t owe me anything.
Having now installed a pretty decent HF antenna system my thoughts turned to VHF/UHF again. I considered another tri-band white stick - but really I didn’t want to have anything bolted to the new house that would require ladder climbing. Heights do not bother me overmuch. 
Years ago I attended BT’s Kew Training School and we learned to climb telegraph poles much like in the photo.
What concerned me was as decrepitude gets ever closer - a ladder accident really needs to be avoided more than ever!
Also, instead of reaching for the credit card, I like to look around at the various “treasures” that followed me from Dartford - and wonder what I could make that would get me on the air.
Two items were selected. An inexpensive nine metre high telescopic fishing pole and some 450 ohm ladder line feeder.
That combination made me think of the Slim Jim and some googling found the excellent web site of John M0UKD.
John has a “Slim Jim and J Pole calculator” which documents all you need to know in order to construct your antenna. The advantage of using the slim jim was that my fishing pole would easily bear the weight of the 1.5 metre length of ribbon feeder.
There are many tips on John’s web page so it bears close study. Which I confess that I didn’t do very well. Therefore I had problems that I certainly could have avoided.
The calculator defaulted to conductors with a velocity factor of 0.96. So a total length of 151 centimeters was predicted (for 145.5 MHz). I found that the completed antenna, (2 metre) SWR was not coming down to a reasonable figure - and further tests showed that the antenna was actually resonating down near 143 MHz.
So clearly it was too long.
Had I read John’s instructions properly, I should have used his suggested Vp figure of 0.90 (instead of 0.96) and then I would have arrived at a length of 141.7cm.
Once I had fixed that mistake then it all started to look better.
John’s drawing shows where the coax is attached and a little adjustment to the feed point got me to the right match.
The fishing pole telescopic elements stay extended due to friction - and remember, fishing is usually done with the pole held horizontally. Left unmodified, the pole sections would collapse down when vertical - and with a crash - possibly causing damage.
So I applied the same method as comes with the excellent Spiderbeam pole which holds up the middle of my HF Doublet antenna. Above each section I applied a strip of old bicycle inner tube and clamped it with a hose clamp.
A month later and the pole has not collapsed, and although the cheap fishing pole is far thinner than the Spiderbeam, it seems to be a method that works just as well.
The harmonic relationship between 2M and 70cms means that I can use the Slim Jim on both bands. My TS2000 has separate outputs for 2 and 70. So as shown in the photo I had to use a diplexer to combine both cables - and then that single output feeds into the Moonraker VHF/UHF SWR meter – and from there to the antenna feeder. The diplexer is able to isolate the two transmitters from each other.
Of course, something has to keep that fishing pole pointing skywards. I’m fortunate to have a tree in just the right place and I simply bungee cord the pole in place and this then avoids using a ladder. It also made adjustments easier while I searched to find why the antenna worked better on 143 MHz!
Weatherproofing was achieved by a very liberal use of self-amalgamating tape. This keeps almost everything out apart from sunlight. So you can see that I’ve covered it with a layer of insulation tape which keeps the self-amalgamating tape from deteriorating.
The result is a very inexpensive dual band antenna that perhaps might not be quite as effective as the tri-band ground plane antenna that I used to have. But it does work - and with 5 watts I can fully quieten the Cheddar repeater around 25 miles away and the Swindon repeater 36 miles away.
A shout out is also due to Mike M0MSN who has an interesting and useful youtube channel which (among many topics covered) features a couple of videos on the construction of Slim Jim antennas using ladder line.
Mike has used 300 ohm ribbon - and that would likely be preferable as it is lighter. I would have used this for mine but I didn’t have any!