IIRC, I read somewhere that a buried radial no longer adds effectiveness to a
system with increased length when its input impedance (which varies with
length) approaches its characteristic impedance (which does not). This is
highly dependent on soil conditions and can really only be estimated. 1/4
"physical" wavelength in your context is really a 1/4 electrical wavelength in
air and is meaningless once the radial is placed on or in the ground. Remember
that the VF is affected primarily by the ground characteristics, not just the
wire insulation.
The characteristics also vary with frequency and the dynamics are not the same
at very low frequencies nor mid HF and above.
Unfortunately, a lot of what we "know" about radials and ground mounted
vertical antennas comes from the MW broadcast industry and from the engineering
practices of the 1930s. The magical 120 1/4 wave radials for an AM broadcast
system is (IMO) based on overwhelming the expected variability of local soil
conditions. Broadcast stations use 120 radials not because of any precise
engineering but because you don't have to prove actual performance when you use
that many. In theory you could get a license to operate using fewer, but you'd
have to validate the actual performance with field measurements. Copper is
cheaper.
Not an expert,
Al
AB2ZY
________________________________________
From: TowerTalk <towertalk-bounces@contesting.com> on behalf of StellarCAT
<rxdesign@ssvecnet.com>
Sent: Wednesday, November 30, 2016 1:23 PM
To: tower
Subject: [TowerTalk] radial lengths ...
For the ‘experts’ out there ...
So question.... in ON4UN’s book it is stated in abundance that one should use
typically 0.25 wl radials... the length of course varies with the properties of
the earth and the desired end results but somewhere around 32 1/4 wave radials
seems to be within about 1 db of 100+ of the same length ...
but he also states, kind of “on the side” in one sections only (it seems) due
to the velocity factor attributed to the earth that 1/4 wave is actually
physically only 0.14 waves in length!
So which is it – when it is stated 32 1/4 wave is that physically 1/4 wave or
is it physically say 1/7th wave (and still electrically 1/4 wave)?!
so for example on a 160 meter vertical are we looking at 32 ~130’ radials or
more like 75’ radials (with the end of each of those not having enough current
in them to contribute appreciatively to the current distribution)... ?
I know most will respond with 1/4 wave.... but I’d bet most would be going by
the generic statements of “1/4 wave radials for verticals”... if indeed the VF
makes going beyond about 1/6th wave of very little value (again talking about
32 radials here – not 120) ... then why go through the effort?
if this is (more) clearly stated in John’s book please let me know where.
Gary
K9RX
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