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Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
Ok, so I decided to go with a loop antenna (which I started, but have to wait to finish due to Surgery I had last week) - but I realized something: the info online saif to not use the normal connections on the variable capacitors due to the extreme voltage across the loop, and it's ability to arc-over in the variable capacitor. So it appears to want me to use the two studs for the non moving portion of the capacitors... but I think I am missing something, or I misread it - would this be right? would the moving plates, which are not electrically connected, be able to tune the antenna as necessary?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It sounds like they're telling you to use a dual-section variable cap with the two sections in series. In this case, if the two sets of moving plates aren't connected, you obviously have to connect them. Often they are connected, just by being on the same metal shaft.
The best kind of capacitor for a loop antenna is a vacuum variable. They're expensive unless you get a lucky deal on a used one, but they can handle massive RF voltages and currents.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
My garden is small so I put up a 20m dipole that's fairly inconspicuous; One fishing pole (the bottom 6 of 7 sections of a 7m pole) attached to my shed in the center supports the balun in a 35mm plastic film can, One fishing pole (the bottom 7 sections of a 9m pole) at the bottom of the garden supported by a fence post, and the chimney on the roof support fishing line to the dipole which is made of 0.8mm diameter 'magnet wire' Height above ground is about 8.3m. (0.8 mm is big enough to be seen by birds but too fine for them to perch) I have had good contacts to northern USA and eastern Europe (QTH = central England) and although the whole lot sways incredibly in the wind it's been up for about 9 months with no problems. The very light construction means that IF the antenna does fall down it should cause little or no damage to surrounding properties. New fishing poles are expensive, keep trying for cheap poles on eBay. You can spray paint the poles to 'camouflage' them.
For NVIS on 40m I ran a balanced feeder along the boundary fence and put the dipole along the bottom of my and my neighbours back fences just below the top, out of sight. (with their permission) It's only 5 ft above ground but it's ok for NVIS.
Next I plan to wind wire around the bottom pole to make a large 'rubber-ducky' for vertically polarised 80m use. That should be inconspicuous too.
Local regulations allow 15m for amateur radio masts ......maybe one day
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
A very fine stainless steel wire will be all but invisible against the sky. Of course it is non-ideal, with a higher resistance and reduced efficiency, but in practice you will hardly notice this loss of 1 or 2 dB at LF/MF/HF. Stainless steel won't oxidize to any great degree, but copper will, so whatever advantages copper has over stainless steel will be swiftly eroded once the antenna has been out in the wind and rain for a few weeks.
To carry on the theme of invisibility, there is no reason not to use nylon fishing line as insulators on your long wire, or dipole. If you put a little silicone on it, it will shed water readily and so keep its resistance even in damp conditions.
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HF Antenna thought... maybe use a light post?
