Antenna's in Restricted Spaces

When I was house hunting in Cumbria, I fell in love with a wee cottage in Underbarrow, just outside of Kendal. At the time it seemed perfect; country living, a detached cottage, and a low noise floor. “Great”, I thought, “I can have the Cobwebb and a 2m mast on the house, run a dipole out in the garden” and so on. I was so taken I even bought a Hexbeam and rotator to mast mount at the side of the house.

A quick look at my QTH on QRZ.com shows the first obstacle; the shared garage and parking area between my house and the back garden. (Click on the photos to open a larger version).

You might just be able to make out the overhead electrical power lines supplying the cottages. The shared parking area precludes running a feed line to the garden, whilst the power lines rule out mounting any antenna on the house.

My temporary workaround was to put a push-up 12m Clansman mast in the back garden for the Cobwebb, and operating outside when the weather is fine. It’s not ideal, and I had to install an external hi-gain wireless access point for internet access when in the garden. So, where else could I put an antenna?

I have a Comet CH250-BX vertical, originally bought for the side of the house on a 12m mast – but the adjacent power line carries a risk of cutting off the neighbours power supply if the mast or antenna collapsed. Enjoying a pipe one day, it occurred to me that the “front” garden was far enough away from the overhead lines to ground-mount the vertical, and still close enough to feed with coax. The feedline would run across the driveway, but could be protected with a rubber cable cover and packed away when not in use. A quick guestimation survey later, and… excellent, it’s ‘doable’!

So, how to ground mount a vertical such that I’m not having to disassemble it when not in use, or in high winds: in January, we recorded a wind speed of 100mph! It blew a tree down onto one of my cars, and blew the garage doors clean off!. The obvious solution is a tilt mast, until I found how much they cost! There had to be cheaper way… I had some old scaffold poles, could I use them?

The ground was soft enough to drive a scaffold pole in, and almost firm enough to hold it. Now, how to make a tilt mechanism? That was solved after seeing some swivel scaffold clamps; I had an idea that I could clamp the antenna to a short length of scaffold, in turn attached to the support pole in the ground.  Scaffold clamps were easy to come by in a village with four builders and a scaffolder. Originally I intended to use a swivel clamp at the bottom to provide the tilt mechanism, and a double clamp at the top to lock the pole in place.  In practice, a double clamp is too rigid, and as there is very little gap between the two poles it is impossible to fully raise/lower the antenna when fully lowered/raised. Replacing it with a second swivel clamp solved this; it is easier to fit to both poles, and the swivel allows me to compensate for the not-quite-vertical support pole.

A trip to an electrical store for an earthing rod and some electrical cable, a quick cut round the hedges to clear the antenna area when tilted over, and I was all set. It only took a few minutes to drive the pole in, mount the antenna bottom section for sizing and cut the excess, and attach the clamps.

Would it work? Would the nearby power lines fry the antenna? The rig? Me? Would it work without radials? Lets try… there was no high-voltage arcing from power line to antenna, nor was I fried. The antenna receives well, I could hear stations, there seemed to be little interference. Could I transmit? Tune to 20m, and a contact with a French callsign. Tune to 40m, success, contact with Germany. It was getting dark, so I stopped there.

You might have noticed I only mentioned radials in passing… because there isn’t enough room at all to run radials. But, much of what has been written about radials is, in my opinion, a regurgitated version of someone else’s regurgitated opinion found on the Internet.  Yes, a vertical will work better with radials. No, it does not need 120 radials each 1/4 wavelength long. 1 or 2 will work, 3 or 4 will work better. 10-15 are pretty much optimum; after that the Law of Diminishing Returns kicks in. But thinking about radials for a moment, who said radials had to be arranged in a star-type configuration around an antenna? If antennas can be bent, such as the Hexbeam and Cobwebb, so can a radial. And if an antenna can be circular or helix shaped, such as TakTenna and VHF special-to-purpose antennas to receive weather satellite data , so can radials. 

The whole point of a radial is to improve the ground conductivity, as the ground mounted vertical is an image antenna with the image half being the ground it is fed against. So, my idea is to lay chicken wire over the garden, thus creating a sort-of ground-plane, and then run three or four radials out in a spiral, as far as I can, all under an inch or two of manure.  Although I could mount the antenna higher, sloping radials would be a waste due to limited space, and the juxtaposition of the road; I wouldn’t want an inquisitive rambler to touch a radial carrying a high PD! I also need to look at whether insulating the support pole makes any difference or not in operation, although I’m not sure how I could achieve that.

 

Now I was looking at my porch, having a pipe, when it occurred to me that it would make a nice little summertime shack…. another day! In the meantime, I’m going to make use of the copper earth rod I banged in at side of house for a random long wire from the office window to the back garden!  Next, a random long-wire from the office window area to the back garden, passing under a mains supply line, and capable of rapid erection and disassembly.

 

Posted: 6 February, 2015

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