Topband: A question on L's

[Tom Rauch]

> Everything I read told me I should be about 30 Ohms or so 
> at the base
> and have a relatively small bandwidth (maybe 30 kHz or 
> so). Not
> complaining about having a flat match or the wide 
> bandwidth, but I
> have to wonder why I'm so out of spec with this, and if in 
> fact I'm
> heating up the ground more than I am pumping a signal into 
> the ether.

Peter,

Some of what you are reading is not correct.

First, the bandwidth of a perfect Inverted L at reasonable 
height is twice as wide as a dipole made from the same wire 
size. This is because the length to diameter is effectively 
doubled. You have HALF the antenna length. 30kHz is 
unrealistic unless we are doing something foolish with the 
ground system like making it small and resonant.

Second, the base resistance in the real world can be all 
over the map without any correlation to efficiency. Roy 
Lewallen, W7EL, was here several months ago and we measured 
a 40 meter vertical under various ground system 
configurations. With some smallish buried radial systems we 
had over 50 ohms of base impedance, but the efficiency was 
the same as with elevated resonant systems having 30 ohms of 
base impedance. There was very little correlation  between 
base impedance and efficiency.

We have a lot of articles floating around that initiate new 
theories or reach profound conclusions, but they never 
actually measure what we really need to know! There can be 
several pages of official looking data with charts and 
graphs....but not a single measurement that actually looks 
at what the article claims.

It almost seems silly to have to say this, but if we want to 
know field strength change we have to measure the field 
strength change. We can't do it with antenna or radial 
currents, we can't do it with base resistances alone. They 
simply don't measure what we need to know.

If we want to know propagation loss we have to know the 
transmitter ERP at the desired angle or range of angles and 
the overall receiving antenna gain and pattern. We can make 
a thousand measurements and none will prove anything unless 
we know that. The same is true for transmitter field 
strength, we have to measure what we want to know....not 
something we guess or assume tells us what we want to know.

Not measuring what we should measure often makes a great 
sounding article, but it wastes a tremendous amount of time 
for the experimenter and the rest of the world trying to 
learn something useful.  Unfortunately even our best 
publications are full of this type of mistake.

73,
Tom