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PA2RHB > WANTED 23.09.04 21:05l 51 Lines 1678 Bytes #999 (0) @ WW
BID : 3A0043PA2RHB
Read: GUEST
Subj: Re: 70MHz Horizontal Mobile Ant?
Path: DB0FHN<DB0FOR<DB0SIF<DB0SON<DB0ERF<DB0FBB<DB0GOS<DB0EEO<DB0RES<ON0AR<
PI8WFL
Sent: 040923/1956Z @:PI8WFL.#NH1.NLD.EU #:48412 [Enkhuizen] $:3A0043PA2RHB
From: PA2RHB@PI8WFL.#NH1.NLD.EU
To : WANTED@WW
X-Info: uploaded with password authentication at PI8WFL.#NH1.NLD.EU
Andy, G0FTD, wrote;-
> 30cm over a car body, low angle radiation ?
>
> You'll get most of the radiation going upwards, and tailing over at
> about 60 at the -3db point.
>
> Very good for working satellites above your head ;-)
I am well aware of calculations of this kind (tell me about it, I spent
most of the day making sure a set of Glideslope antennas were at the
correct height in the mast for the desired angle of descent..)
Actually, for 30cm over the ground plane with 4m wavelength, -3 dB is at
about 45 degrees. That is, -3 dB compared to maximum radiation, which is
straight overhead, obviously.
Mind you, _I_ did not say you get a lot of low angle radiation from an
antenna at 30cm over the car top. I would place it far higher, but the
question was, Can we go to 20cm. So I say No, even at 30cm, -3dB is as
high as 45 degs, so do not go under 30cm.
A close approximation for the expected field strength from a low
horizontal radiator is:
E/E0 = 2 * SIN (2 PI * (h/lambda * SIN (theta))
(where theta is measured from the horizon up, not the zenith angle)
(E0 is what you would get without the reflection)
For a 30cm and lambda is 4m, we get
E/E0 = 0.91 for 'zenith illumination' (th = 90 degs)
= 0.65 for th = 45 degs
the ratio is about 0.71 or 3dB.
That E/E0 is only 0.91 at maximum radiation does mean that there is a lot
of cancelling going on, and radiation resistance will be quite low.
That is another sign you really want to go up with your antenna, but for a
different reason.
Rudolf, PA2RHB @ PI8WFL
Heemskerk, JO22im
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