A buried wire is a transmission line. When a transmission line is very short
compared to a quarter wavelength, its usually considered to have practically no
transmission line properties of its own except for a small amount of resistance.
Three primary factors are at play when using a very short transmission line (a
buried insulated wire or coaxial cable) to connect two sections of a Beverage
antenna:
Dielectric loss - a directly buried insulated wire will have more dielectric
loss than a coaxial cable or a wire enclosed in a plastic conduit. A very
short directly buried insulated wire might have insignificant dielectric loss
(it wouldn't be difficult to measure this).
Mismatch loss - a very short transmission line (a buried wire in a conduit or a
coaxial cable) has negligible mismatch loss. When a transmission line is very
short (compared to a quarter wavelength), its characteristic impedance has
little consequence to the circuit in which it is operating.
Velocity factor - minimizing the length of a coaxial cable, using high velocity
factor coaxial cable (such as cable TV hardline), or using a very short buried
wire in a conduit will minimize the phase delay between the two interconnected
segments of a Beverage antenna. Using tennis balls to center the wire in the
plastic conduit as recommended by KV4FZ will further reduce the phase delay of
a buried wire.
73
Frank
W3LPL
---- Original message ----
>Date: Fri, 06 Jan 2012 12:33:09 -0400
>From: Herb Schoenbohm <herbs@vitelcom.net>
>Subject: Topband: Fwd: Re: Bev wire under a path?
>To: TopBand List <topband@contesting.com>
>
>
>
>
>On 1/6/2012 10:33 AM, Ryszard Tymkiewicz wrote:
>> <Has anyone any experience of the effect on performance. I will be burying
>> <approx. 2-3% of its total length.
>>
>> Neil& All,
>> I was using this kind of passing roads quite often....usually the distance
>> was 3 meters in a soil. It was not affecting 160m but on 80m I noticed
>> a difference , probably a capcitance to a ground was significant.
>> In my main QTH where I have a lot of houses and tracks around
>> I'm using thin wire 0,7 mm and a support in 50 % are just trees 3-7m high.
>> Its easy to throw a reel of such wire over a tree.
>>
>> 73 Rys
>> SP5EWY
>
>
>
>If looses are noticeable due to ground effect trench attenuation you can
>use a bigger conduit and a bunch of tennis balls with a hole drilled in
>the center as insulators to greatly improve the VF in an underground bridge.
>
>Just a thought.
>
>Herb Schoenbohm, KV4FZ
>
>_______________________________________________
>UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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