antenna-theory.com

[kf5hce]

I would like to set up an end-fed long wire antenna around my back yard fence. I am going to use an antenna tuner and less than 50 watts of power. Here is the question: when attaching the wire to the fence, does the "insulator" matter? Can I use rubberized metal eye hooks? I am talking about the types of hooks you see at home depot that are used to hold up small items. Or a better description.... you know those hook and loop closures for gates.. the ones where the gate has a long straight hook and the fence has got a small hoop for the hook to go into. Can I use those loops along the fence to hold the long wire up? Will the metal hoops running perpendicular to the wire antenna act like traps and prevent the antenna from functioning?

Thanks.

[admin]

What frequency are you running at? I think you will be ok

[k3nnw]

I agree with the previous response: no, the insulator won't matter.

Assuming your antenna is used at frequencies below (e.g.) 50MHz, we can show that the change in performance of the antenna is a significant function of height above ground, the nature of the ground, and the shape of the antenna (straight, or bent through 90 degrees at some point(s) i.e. routed as a low-order meander). There will be a change in performance on account of the fence type, and if it is wet or dry due to the weather.

the effect of the insulated eyes (e.g. the mutual inductance between the driven element - the long wire - and the loop of metal being a parasitic element of some sort) will be minimal and will not stop the antenna from performing. There is some possibility that, with much higher drive powers into an antenna of arbitrary length, higher voltages might present themselves at positions which at some frequencies correspond with the insulated eye(s) at one or more points and there coulld be leakage currents to ground through the eye(s) and the fence, but we might assume that the leakage path's resistance is high enough not to cause significant current flow (via the long-wire insulation, the rubber "eye" insulation, down the metal of the eye into the wood, down the wood to ground etc.).

A simple concept to start with, in other words some initial qualitiative analysis, would be to consider the dimensions of the wire where it passses through the "eye", in terms of asking what portion of a wavelength at the highest operating frequency is the geometry? At 50 MHz, lambda = 6 metres in otherwords an inch (say) is only approx. 4 thousandths of a wavelength. Not much! Thus the section of wire leading through the eye can be considered as a lumped element and will have minimal effect on the driving point impedance of the antenna..

You could go on and determine the inductances of the wire and the eye and thus derive the mutual coupling M. If it was my fence and my garden, I would certainly not hesitate to just try it with a drive of 50 watts. I am certain it will work just as well with the eyes present or absent.

As an aside, some modelling in e.g. 4NEC2 might prove interesting so that you can seee the effect of the antennna on top of a fence (e.g. 1.8 m or 6' above ground compared with something significantly (10x) higher) and the effect of changing the type of ground (mainly its conductivity).