----- Original Message -----
From: "Kevan Nason" <knason@emeraldis.com>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Sunday, August 11, 2002 12:47 PM
Subject: [Towertalk] 160m Inverted L
>
> Thanks to those who responded. Suggestions were to increase the
> top length until Rradiation is 50 ohms or to leave it as is.
Rr has NOT to be confused with the resistive part of the complex impedance,
in the series form, that one you're actually reading.
In other words, if the resistive part of the complex impedance would express
Rr itself, then adding a resistor would give you the chance to rise it.
Actually, in an antenna Rr can be anything but losses should be accordingly
minimized.
In
> both these cases it of course is necessary to add a series
> capacitor to cancel inductive reactance. It was also suggested to
> add more radial wires. That would (perhaps) lower radiation
> resistance more and require a matching network.
>
> I woke up at 2 A.M. thinking "AHA!" The answer is I need radiation
> resistance to be low so that I get more current flow and a stronger
> field strength.
You've the maximal current when you match the impedance and you keep down
losses.
In your case, the point You've to care about are losses, then deal with
ground.
What you measure with your instrument as R, is the sum of the antenna R plus
ground losses.
The reason why you should try to lower your R has sense only if you deal
with lowering ohmic losses that are summed to the antenna resistance.
Leave antenna lenght as is for now, put radials and/or rods, anything that
lowers the measured R is OK.
That means (in this case of an L antenna) I should
> go for resonance which will keep the Rrad low and thus increase
> my field strength. After all, that's what all the antenna guru's
> try for by laying 120 radials around a vertical. It seems they value
> the field strength gain over the (mathematical) antenna efficiency
> gain of keeping the radiation resistance near 50 ohms.
Who deals with ground planes does it to lower ground losses, when a
quartewave GP shows about 37 Ohm resistive and zero reactive, for example,
it means that the ground plane is almost perfect.
> But as one person mentioned the horizontal length helps for local
> work so maybe shortening that more isn't such a good idea.
The horizontal section can influence with high angle radiation only if a
significative current is flowing on it.
Moving the current maxima along the antenna can be, or can be not, a good
idea.
Depends
> on what you want.
>
> Decided to use a series capacitor and leave it as is. It should
> be a pretty close match SWR wise.
A better idea is to minimize ground losses as much as you can, the leave
antenna as is and do something with your ground plane trying to lower the R
you read on the instrument.
When You've done it you can lenghten (or shorten) your antenna as you like.
The idea of a capacitor in series (keeping the antenna inductive) could be a
good trick if you do it variable, and remotely operated. This will help You
to cover easily the whole band.
73,
Mauri I4JMY
> It also occurred to me this is probably just a rehash of the old
> question of 'is it better to match an antenna directly or use a
> matching network?' I guess that's a personal decision and I really
> don't want to open that up again. It didn't occur to me yesterday
> that is what this post is really about. Things often become clearer
> the next day. Reminds me of a school lab where I stumbled on a neat
> relationship between voltage, current and resistance. Spent an hour
> proving it. Called the instructor over and he said "Congratulations.
> You've just proven Ohms Law."
>
> Kevan
> NS4T
|