KV9U wrote: > Paul, > > A full sized 160 loop is not easy to set up since it needs to be around > 500+ feet in circumference (1005 / 1.9 = 528 feet). This would make a > square about 130 feet on a side.
Yep. I was thinking of the 80 meter one. At 130 feet or so per side, it's pretty big, but still fits in some lots where there's not enough length to do a 160 dipole. I really should restrict things that require thinking to before 5:00 PM when I switch the brain in standby after work... > After comparing a nearly full size, low height (30 foot apex) 160 meter > inverted vee to a very low (10 to 30 foot) "L," the "L" was good enough > that it worked for NVIS operation. It is directly connected to a > multiband 80-6 meter ground mounted vertical but has quite a few radials > up to 100 feet long and in the clear. > > If you use a dipole that is much smaller than 75% or so from full size, > even with open wire line you can expect some substantial losses in the > tuner. For example, using smaller than 180 feet for a 160 meter dipole > can be lossy. It might be better to use linear loading or lumped > inductances in the antenna. There are always compromises. My dad's running a 75-meter inverted vee fed with open-wire line and an LDG auto-tuner. It works quite well on a 100-mile path near the top end of 160 meters. (It also works quite well at resonance until it's too long after dark and there's no more NVIS propagation that high in frequency!) There are some formulas out there somewhere for computing the necessary loading coils to make short dipoles - I used to have an old DOS program that would do it. Google will probably find an up-to-date version - and as you state, that's probably the best solution. Put up an inverted vee that's as long as the lot allows, and put a loading coil in it to resonate it on 160. Add a tuner to get some reasonable bandwidth and to compensate for the <50-ohm feedpoint impedance. > There are some hams giving out misinformation on low dipoles for NVIS > operation, particularly on the NVIS yahoogroup. They believe that low > dipoles only a few feet above the ground will give you a better S/N > ratio and will actually work better for NVIS. I have done experiments > with 80 meter inverted vee dipoles at 12 feet apex and 35 feet apex and > the higher dipole is consistently superior than the lower dipole with > just about any measurement you want to use, received signal strength, > S/N ratio, and for sure, transmitted signal strength. There *is* a limit to what "low" is for it to work. Get too close to ground, and run into trouble. A 35 foot apex on an inverted vee is still considered low (1/8 wavelength at the top) And at 160, a 35 foot apex is even lower (in wavelengths). My dual-band 80/160 inverted vee is about 40 feet at the apex, and works quite well for NVIS on both bands. > The higher your horizontal antenna can be and still be below 1/4 > wavelength, the stronger your NVIS signal. This means that you would > have to exceed 60+ feet to be "too high" on 80 meters. Double that for > 160 meters and half it for 40 meters. Needless to say, many of us have > NVIS dipoles on the low bands without even trying:) That's a fact that's most often overlooked on the NVIS group. They're always asking things like "how do I use the NVIS mode?" when the fact of the matter is that if they're communicating at all at the frequency and distance they're talking about, they're already doing it :) NVIS has become somewhat of a buzz-word, when it really shouldn't be. It's not magic or anything super-special or complex. It's simply how things work and have been working for all those years before it became a popular subject on internet discussion groups... Thanks for the correction on the loop size... 73, - ps
