October 16th 18, 09:59 AM
posted to uk.radio.amateur,rec.radio.amateur.antenna
external usenet poster
First recorded activity by RadioBanter: Jan 2014
On 15/10/2018 16:45, Jeff Liebermann wrote:
On Mon, 15 Oct 2018 12:16:14 +0000, Spike
On 15/10/2018 01:20, Jeff Liebermann wrote:
On Sun, 14 Oct 2018 11:12:14 -0700, Jeff Liebermann
Since you prefer a minimalist approach to test equipment, as an
alternative to your light bulb, may I suggest a return loss bridge:
Note that there are several basic designs and configurations but all
are fairly simple and easy to construct. Note that these are NOT the
same as directional couplers.
To use it, you need a minimum of an RF signal generator and a
voltmeter or oscilloscope. I prefer to sweep the frequency range of
interest, so I use an RF sweep generator, and display the result on an
oscilloscope. With this arrangement, you can tune your antenna
without requiring a light bulb.
So, let me get this right. By employing a return-loss bridge, an RF
signal generator, and either a voltmeter or an oscilloscope, you can get
results that a distant station can't distinguish from those obtained by
using a torch bulb?
No. Per my previous rant, if your intent is "to be able to transmit
signals intended to be received by another station", then a light bulb
will suffice at producing the desired result. If your intent is to
design the best possible antenna, then you'll need something better.
If you just want to talk to someone, almost any kind of RF metering
device is sufficient.
There have been plenty of accounts of comparing various types of
antennas. For example, PSK Reporter is a good way to perform such a
test, where one can actually see the effects of antenna changes.
What I've found is that such side by side comparisons do not account
for variations in propagation, path, interference, local noise, time
of day, position of the moon, and other factors beyond the operators
control. A given antenna might be far superior under one set of
condition, and rather disgusting under another. Most signal reports
also tend to be very subjective, inaccurate, and not repeatable.
If you are using a light built to tune a commercial antenna, which has
already been optimized in extensive lab and field tests, I suspect
that it is likely that a light bulb will give a similar result a
proper VSWR measuring device. (Actually, that's not quite correct
because I don't tune my antennas for minimum VSWR). However, that's
not why someone purchases and uses a VNA or swept return loss bridge.
They use these because they're building their own antenna, or
optimizing a commercial antenna. Once the antenna has been properly
tuned and tweaked, the VNA and return loss bridge are no longer needed
unless something changes.
Incidentally, I use a remote field strength meter to compare antennas.
It has it's limitations, but it's better than using VSWR or maximum
antenna current as in your light bulb method.
Given your ability to estimate the performance of an antenna by looking
at it rather than employ modelling methods, I would have though you
would be sympathetic to the merits of the torch bulb approach.
Since you seem impressed with my powers of observation, it might be
useful to know that to the best of my limited knowledge, light bulbs
went out of fashion in the 1930's, to be replaced by thermocouple
antenna current meters.
It is much easier to see changes in a meter deflection than changes in
light bulb intensity, unless you also use a light meter. If you
select different light bulbs for different power levels, you might be
able to keep the losses to a minimum.
In any case, a VNA or even a return loss bridge is not for you. There
are plenty of things one can do with ham radio including "to be able
to transmit signals intended to be received by another station". You
seem intent on using the oldest and most crude methods of
accomplishing this. That's fine as there is room for retro-radio,
antique radio techniques, and preserving historical technology. I
would guess(tm) that your radios all use tube (thermionic valves) and
that you tune the transmitter for maximum cherry red glow in the
finals. Best of luck, but that's not for me.
Very interesting, but I'd have to say that none of what you say refutes
my original contention that the distant station, which after all is the
one we are trying to communicate with, will notice any difference to the
received signal whether the sending station's antenna was tuned with a
20c torch bulb or a $300 VNA. You touched on the main vagaries of the
system when you said "What I've found is that such side by side
comparisons do not account for variations in propagation, path,
interference, local noise, time of day, position of the moon, and other
factors beyond the operators control".
STC / M0TEY /