TheRadioBoard

Now that I've successfully managed to sometimes tune my small transmitting loop antenna to a low SWR, I need to measure the antenna's -3dB bandwidth to get some idea of the losses in the antenna. (There are limitations to the notion of equating low bandwidth with high efficiency, but let's ignore that for now.)

What's the easiest homebrew way to do this without causing undue QRM? Right now I can think of a couple of options:

1. Build noise bridge, calibrate it, and null the bridge while tuning the receiver in 2.5kHz increments. Make a plot of the R and X vs. F That would show me how the antenna's complex impedance changes over the band, but is there a way to use the frequency-varying complex impedance to compute the antenna bandwidth?

2. Build a 30dB attenuator, and run the rig at 500mW output power, for 0.5mW (500 microwatts) output power, and just sweep the rig up and down the band and plot the SWR bars (which requires converting the bars to actual SWR readings, which may not be easy). But will 0.5mW be enough RF to generate an SWR reading? Is 0.5mW low enough not to cause undue QRM when sweeping the band?

3. In combination with 2, build a homebrew SWR bridge to get actual SWR readings instead of the nebulous "number of bars" that my rig outputs. But how to calibrate the homebrew bridge? Is it feasible to do the calibration manually?

Any comments, or other ideas?

Thanks in advance.

The standard method is to determine the SWR = 2:1 bandwidth, above and below SWR = 1:1. Don't sweat the QRM; how long is it going to take? If you're really worried, do the tests when the band is dead.

73,

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http://kr1s.kearman.com/
http://qrp.kearman.com/

+1 to what Jim says. If you really wanted to measure the 3db power out point I imagine the easiest way would be a remote field strength meter or fellow nearby ham with an s-meter.

OK, well the next issue is then how to accurately measure a 2:1 SWR when all that my rig displays is some fuzzy relative SWR indication.

It looks like a return loss bridge might be easy to build, calibrate, and use for accurately measuring SWR:

http://www.vk2zay.net/article/179

As for avoiding QRM, I am concerned about sweeping the band (with key down) while watching SWR since I might sweep right over a QSO in progress. I suppose the politer alternative is to take SWR readings at several individual, closely-spaced frequencies, always listening for a minute on each frequency before making a brief transmission to check SWR (and of course skipping any in-use frequencies).

I'm lazy, If I didn't already own one, I'd check and see if a local Ham would let me borrow his analyzer.....

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73 De KD8DEY

Oh, good grief. If the receiving station even notices, he'll probably think you're an ionosonde. If we all waited until our Morse Code was perfect and every aspect of our stations was documented before we went on the air, the ham bands would be as quiet as the tomb. Time to make some QSOs!

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

qrp-gaijin -- my homebrew VSWR bridges for 300 (& one for 50) ohm lines work pretty well for measuring the VSWR vs frequency. Slow going but they work.

http://theradioboard.com/rb/viewtopic.p ... sc&start=0

But I like BubbaTheGimp's idea. It would be great fun to have one of those antenna analyzers while fiddling around with an antenna.

Enjoy. 73,

Dan

I'm getting there soon, really. However on my loop antenna, the butterfly cap's solder joints (connecting the tarnished cap to the loop conductor) look really ugly, and I fear significant ohmic loss there. Maybe it would be easier just to clean the cap, re-solder the connections, and try to make some QSOs instead of measuring the antenna loss.

Is your iron hot enough? You may need significant heat to solder to those posts.

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Good point. I used a 30 watt iron, and it was hard to get the solder flowing even after I had tinned the posts.

Time to dig out the 100 watt iron, which I bought long ago but haven't yet needed.

Edit: the solder joints after reflowing with a 100W iron and plenty of silver solder. Still looks pretty ugly, but I think the solder has flowed cleanly.

it looks workable, the idea for lowest loss is minimum amount of solder to do the job.

let us say that you solder a wire to a PCB you should be able to count strands in the wire through the solder and solder should not spread much more than 3x with of wire from side to side.

there is a trick that may work in some situations, use a permanent marker to make a "dam" to hold the solder in place some and prevent it from spreading, in practice you are making controlled tarnish, preventing flux to work well, this also helps some with keeping the flux from contaminating unwanted places, then just wash away with alcohol and not much of a trace is left.

That's very interesting. Why is minimum solder better for lower loss?

Pick up any reference book and have a look at resistance of solder as opposed to copper, silver...
The trio below taken from tables I found in old text on "radioteknik för el tele" a school text from 1950's,

likely solder has improved since the table was printed, still, oxides of lead and tin are NOT good conductors...

Oh, this is solder alone without alloying barriers and such.
if you use solder to first bond silver with copper, you get regions between solder and base metal that are "barriers" elevating resistance some each transition layer...

If the solder was higher resistance, then more of it, covering a wider area on each part of the joint, should add up to a lower resistance joint than a minimally soldered one would? (thinking logically?)... but .... maybe not, in HF situations.

BTW (FWIW) in the old days when terminating wires on tags in telephone exchanges we were told to wind the wires tightly around the tags, as the main connection was between the wire and the tag, and that the solder should mainly hold it in position and seal it against oxidization.

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