MFJ Loop

[drskeptical]

Hello
This may have been asked before. Sorry if it has.
I have just bought an MFJ loop. It is the 1788X version for my QRSS experiment on 10.140MHz.
Just looking at the way the loop tunes up, I see we use the coax to carry the dc motor current for the tuning capacitor.
I was thinking that I should tune the loop in the normal way, then remove the MFJ tuning box and measure with the 4170.
Only down side here is if I want to "fine" tune the aerial I have to reconnect the tune box and keep switching over. Pain in the !
Any one tried this before?
All information will be appreciated.

Andy

[Bob]

The RF port of the AIM is dc coupled, but you can make an adapter with a blocking capacitor and two diodes. It should be mechanically rigid so you can calibrate with the blocking circuit in place and then get repeatable results.

For 10MHz, a 0.01uf cap would be big enough. It should be able to stand the control voltage. Put back-to-back diodes from the cap to ground on the side that connects to the AIM. Use computer diodes, like 1N4148's. These will clamp the transients that feed through the cap when the antenna and controller are connected or disconnected. It is also a good idea to put a large resistor, like 1Meg ohm or so, across the cap so static charge doesn't build up. The small dc current that flows through this shunt resistor will not affect the AIM.

Make sure the dc control voltage is clean, without AC ripple or noise spikes.

With the blocking cap in place, do the usual calibration with the short, open, and resistor load on the output of the cap where the antenna will be connected. This will compensate for the strays introduced by the blocking cap.

73/Bob

[drskeptical]

Hi Bob
Many thanks for the info. I will knock this adaptor up over the weekend and try it.

Loop arrived this morning, so I should be able to test this weekend.

Many thanks once again for the quick response.

73 Andy

[drskeptical]

Hello all.
Well the adaptor works ok. I have put the loop up tuned it and tested. Great stuff.
Thanks again Bob for help and advice.

If you want to you can click here www.amsnw.myzen.co.uk/qrssproject.html

Scroll down to the bottom of the page to see results.
I am impressed with both the aerial and the AIM4170 again..

73 / Andy