Well, when you get down to it, a quarterwave whip above a quarterwave radius
ground cone is really much like a halfwave dipole, only less directive. the
cone gives it 50ohm nominal impedance but they both radiate in a doughnut
(circular toroid?) shape centre on the horizon - if the dipole is vertical
like ours. The cone just centres the pattern more accurately. I don't think
there's an 'above' or 'below' factor otherwise.
More intellectual minds may differ........
Ferg
----- Original Message -----
From: <n3eu@comcast.net>
Subject: Re: Europa-List: RE: Com Radio Antenna Problem
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| Jim Puglise wrote:
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| > There are multiple grades of coax and you want to use high grade coax
| > and "type N" fittings.
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| I can't agree that an "N" connector verses BNC will add anything at VHF,
Jim, though it would be better where it's subject to environmental
contamination. I've seen specs on factory male-female BNC connector pairs
at a nothing .2db insertion loss, and even if user-fabricated connectors -
true of any connector, any additional loss is still very small.
|
| At VHF also, any RG-58/whatever at less than 20 feet is perfectly
adequate, though solid conductor is arguably not good mechanically.
Lower-loss cable (400, 142) will be specified at transponder and GPS
frequencies, though.
|
| > The other thing I would do is build a simple
| > vertical dipole and jury rig a piece of cable to the radio and try it.
|
| The interesting thing about that is whether a 1/2-wave dipole behaves any
different from a 1/4-wave monopole whip antenna, if in free space (an
airplane). You'll find long/lively threads about this on ham newsgroups, so
I once posed the question to an actual engineer from an actual aircraft
antenna company. He said that despite the stuff in many texts on antenna
theory, they're exactly the same well above the earth. At least it mostly
answers the question as to why airframe mfrs don't mind comm antenna
installations on the top of fuselage, contrary to the insistence of the
1/4-wave "half-donut" purists that it shouldn't work well at all where an
ATC antenna is perforce below your altitude.
|
| It's also been written that a dipole is more sensitive to interference
---From nearby metal elements than a monopole whip mounted inside a plastic
airplane, but if my antenna engineer friend is right, that might not be true
either. Maybe like the three rules of real estate valuation - location,
location, location!
|
| Regards,
| Fred F.
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