Topband: Fwd: Re: Bev wire under a path?

[donovanf at starpower.net]

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 at vitelcom.net>  
>Subject: Topband: Fwd: Re:  Bev wire under  a path?  
>To: TopBand List <topband at 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