TheRadioBoard

Using a noise bridge to tune an antenna is done, I believe, by setting the bridge for 50 ohms resistance and 0 ohms reactance, then tuning the antenna (e.g. adjusting an antenna tuner, or trimming a dipole) until a noise null is achieved.

However in the case of a small loop antenna, tuning the antenna's capacitor for resonance will greatly peak the noise that the receiver hears from the antenna.

It seems that these effects may work against each other, i.e., as you are adjusting the antenna closer to resonance (resistive impedance) and trying to reduce the noise from the bridge's noise source, at the same time the noise from the antenna is coming up as you approach resonance, which may mask the noise source's noise and make finding the null difficult.

Is it workable to use a noise bridge to tune a loop antenna?

I don't have a noise bridge, but was planning on building one for the express purpose of tuning my loop antenna.

It's a tedious process, which is why you don't see noise bridges around much anymore. Devices like the MFJ antenna analyzer (Comet and others make them as well) are easier to use because they generate a signal and display the load reactance and, on some analyzers, its sign. For the loop, I'd adjust for max band noise, then use the transmitter and its SWR meter at low power, to get the final match.

73,

_________________

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

OK, well even if the loop antenna is most easily tuned by peaking the band noise, I'd still like to build a noise bridge nonetheless. It seems like an interesting instrument. It occurred to me that I could use a noise bridge to answer my previous question about transmission line impedance mismatches: null the bridge with a 50-ohm resistor connected directly to the bridge, then do the same thing with the resistor connected at the end of a run of 50-ohm coax, then do the same thing after adding a short 75-ohm run of coax and the RCA cable adapters, and see how the impedance changes in each case.

As for using the noise bridge with the loop antenna, I found some noise bridge designs that modulate the noise generator noise to make it more distinguishable from band noise. Perhaps that might help in tuning up a loop antenna near resonance.

Though it might not be that important for practical tune-up (where, as you say, you can use the rig's SWR meter), I'd really like to get some confirmation that my antenna is somewhere near 50 ohms before letting my transmitter touch the antenna. Also I like the idea of tuning up without causing QRM, however brief.

Modern transmitters have high-VSWR-foldback circuits that reduce drive proportionally to SWR. What will kill a solid-state rig is overtemperature, which can be caused by endless twiddling trying to achieve perfect 1:1 SWR at too-high power levels. PA modules have built-in temperature sensors, but modern rigs generally have inadequate heat sinks, to save weight and cost. The PA transistors may sacrifice themselves to save the sensors. AM rigs were designed for 100% duty cycle. Not so the rigs of today.

Believe me, with that antenna, the few tens of mW needed to measure SWR aren't going to cause QRM! But if you can find the components, by all means build a noise bridge. They're fun to play with.

73,

_________________

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

That probably explains Mike Underwood's claims about excess magnetic loop antenna efficiency. He is maybe only putting 10 Watts into a given loop antenna but thinks he is putting in 100 Watts.

Modern rigs usually have power and SWR metering built in. Qrp-gaijin's rig is tiny so its meter isn't large enough to read anything but relative SWR and power. A 100-W transceiver should tell the operator what's really going on. I'll agree though, that the efficiency numbers for small loops seem optimistic at times!

73,

_________________

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

While we're on the subject of small transmitting loop antennas: if I am not mistaken, you earlier proposed a two-turn foil loop on another group:
http://dir.groups.yahoo.com/group/loopa ... ssage/7445

If you have done experiments in this area, do you have any results you could share? Did you try a two-turn strip loop?