[TowerTalk] radial lengths ...

[Al Kozakiewicz]

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
 
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From: TowerTalk <towertalk-bounces at contesting.com> on behalf of StellarCAT <rxdesign at 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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