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 <[email protected]> >Subject: Topband: Fwd: Re: Bev wire under a path? >To: TopBand List <[email protected]> > > > > >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
