Posted by
Guy, K2AV on
Aug 28, 2010; 6:07pm
URL: http://elecraft.85.s1.nabble.com/Need-K3-dB-Measurements-on-new-160M-Antenna-tp5468244p5473204.html
Alas, after years of doing A/B manual coax switch tests, I have given
up on that exposing anything better than 10 db differences on anything
other than stable local signals. And that is suspect because it is
often ground wave, which bears no resemblance to sky wave.
I built a 60 hz switch which used 12VAC plus and negative to
alternately bias off left and right diodes connecting a common RF
output port to two RF input ports. Being careful to use no AGC or
very slow AGC, this presented an audio from the RX which showed to an
oscilloscope the signal comparison between A and B that could be
measured on the scope and converted to dB.
I also quickly learned that I had no hope whatsoever of perceiving a
difference less than 3 dB in my ear and didn't do all that well with
less than 6.
I used that to compare signals on various antennas and showed it to
the owner. But so ingrained is the idea of manual A/B coax switching
that he was back to judging results the old way, and discarding
methods that gained a dB here and there, because "he couldn't hear it,
and was going to trust his ears."
The main problem of the device was an unambiguous way of identifying
the port on the oscilloscope display. I have an idea of using a PIC
device instead of the house AC to create the switching intervals, one
which starts a sequence with a "long" A port and ends with a long B
port and 8 regular ports in between, with a space between the two long
ports. That would always unambiguously identify the A and B signals.
Follow that with a program to analyze the audio levels and present
peaks, minimums and averages for both signals and signal-to-noise, and
you now have an antenna analyzer that can show you real differences
between antennas real-time.
To me anyway, that sounds like a tailor-made Elecraft gizmo kit. I
think you could sell tens of thousands of those. Really surprised
something like that not already around and part of during-contest
comparisons between antennas.
If the gizmo had the ability to decode the results and put it on a LED
display marked with port A on one end and port B on the other, with
the middle LED meaning equal, with two or three ranges, it would be
the cat's meow for comparing two antennas.
73, Guy.
On Sat, Aug 28, 2010 at 11:49 AM, Graham Kimbell G3TCT
<
[hidden email]> wrote:
> Jim
> Afraid I can't help with listening on 160, but an interesting topic, and
> a measurement technique applicable to other situations. My observation
> is that the mental averaging will be rather tricky, especially if the
> QSB is slow and unpredictable. What we need is something (software?)
> which will record the strength seen by the K3 over a minute or 3, and
> then provide a statistical analysis. You would then get a picture of the
> average, peak, trough and distribution of strengths.
> I wonder if something like this exists already - or could be included in
> P3 s/w?
>
> Graham
>
> On 19:59, Jim Brown wrote:
>> Crew,
>>
>> I've put up a pair of new antennas for 160M that are predicted to have
>> about 3dB of gain over my existing vertical. Without going into a lot of
>> detail, each antenna is a wire sloping off of my 120 ft tower, one going
>> east, one going west. The tower acts as a reflector. The tower and each
>> wire have 4 elevated radials. The antenna is working -- I've made four QSOs
>> with VK, one with FK8, and one with FO in about two hours -- but I need to
>> figure out if it's working better than my existing omni vertical.
>>
>> By doing a lot of listening, I can clearly confirm the predicted 6dB or so
>> of front to back, but QSB makes it hard to get a handle on gain, and I'm
>> only expecting 2-3dB. That's where the dB meter in the K3 comes in handy.
>>
>> My method is to key down on one antenna for a while, let you get a reading,
>> switch to the other and do the same, then back and forth between them,
>> again for long enough for you to get a good reading. There IS a lot of QSB
>> on the band, so you'll need to do a lot of mental averaging. Please let me
>> know, OFFF THE LIST, if this is something that you would like to help me
>> with. I need reports from stations that are between about 30 degrees
>> azimuth and 120 degrees azimuth of my QTH south of San Francisco. In other
>> words, my antenna is aimed at about 75 degrees (ENE), and I need reports
>> that are within 50 degrees of being on axis. I could also use measurements
>> from KH6 of the antenna that goes in that direction.
>>
>> BTW -- to access the dB meter in the K3, you need to hit the Display button
>> once, then rotate the second VFO knob clockwise until you see a reading in
>> mV. Let the knob in that position for about 15 seconds, then rotate it one
>> more position clockwise. Now, the dB reading will be relative to whatever
>> the average IF output was when the knob was in the mV position. Note also
>> that the reading goes plus and minus.
>>
>> 73, Jim K9YC
>>
>>
>>
>>
>>
>>
>>
>>
>
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