Because an antenna should never live longer than a week...

26 February 2012 | Comments

Although the majority of my radio operation is out portable, I occasionally play a bit from home as well - both on HF and VHF. Living in a semi-detached house in a residential area, with a back garden just over 40' long brings inevitable compromises for HF antennas...

80m on Blencathra G/LD-008 (IMG_7316)

Initially, I ran a 1/4wave vertical for 40m using a fibreglass pole planted in the flower bed, and a good number of radials dug into the lawn at regular intervals. Nice and simple, but very noisy on receive, and not terribly good on transmit either - most of the radiation went straight into the surrounding houses. Including a neighbour's hi-fi system - not good.

The next attempt was an off-centre-fed dipole for 40m, fed about 1/3 of the way along its length. The feedpoint was supported from the alloy pole that holds my VHF colinear, on the side wall of the house. This fitted very neatly: the short end hooked over the gutter at the front of the house, and the long end dropped to the rear fence. The OCFD gave me greater flexibility in terms of bands (with an ATU) - effectively all of 40-6m were somewhat usable. Since putting it up, the neighbour also reported no further issues with his hi-fi. However, the noise was unceasing (S9), and transmit performance remained poor. Time for something better...

On a whim, I tried making a balanced, horizontal version of the 7.6m "Rybakov" vertical - 7.6m either side and fed with a 4:1 balun. The ATU matched it (but then, the LDG Z-11 will match pretty much anything), but TX performance was well down - probably 20dB. Modelling in EZNEC showed a feedpoint impedance that suggested almost all of the 100w was likely being dissipated either in the tuner or the balun!

Suffice to say, that didn't stay in the air for very long, and was quickly replaced by a hastily constructed half-size G5RV. The G5RV is something of a politcally charged antenna - there are those who swear by them, and those who claim they're little better than a leaky dummy load. My view is roughly as follows:

Given these, I'd done a reasonable job - the ladder line was pulled away from the metal pole, and there was only a couple of feet of coax before it hit the ATU. Performance was pretty good - the noise now at S5, and at least some of the RF leaving the radio seemed to be making it into the ionosphere.

However, the ladder line as it was looked messy from the road, and any good solution I could think of for fixing it more permanently would involve either lots of visual clutter, or introduce significant effort to any maintenance of the antenna. The ATU also seemed a little unhappy on some bands (particularly 30m), and there were some effects I can't explain.

So today, that's come down after only a couple of weeks in the air, and has been replaced by a pair of nested dipoles for 40 and 20m with a common feeder, the former also doing a reasonable job on 15m. The 40m dipole runs between the back fence and a convenient post at the front of the drive, and the 20m is tied off to the gutter at the front and back of the house.

After a previous bad experience with a 10m and 20m pair of dipoles where there was significant interaction between the two when trimming them, this installation was much easier, and they present a good match on both bands.

Noise seems reasonable (S5 or thereabouts); I've not yet tried firing it up on transmit. No doubt in a week or two's time, something else will be up there instead!

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