A Loop for $20 (New Zealand)

[invergordon]

I had a few hours to spare the other day and have had a plan in my mind to construct a loop antenna to replace my wires (dipole, longwire, delta loop etc.) non of which seemed to perform very well recently probably due to an increase in local noise. I'm an aero nut and have always had an interest in longhaul aircraft communications.

So for NZ$20, which was the cost of a few secondhand variable capacitors I made a 1m diameter loop and found that it performs better than any of the wires I've strung up before.

What I used:

A wooden cross to hold the loop in place (ex-building frames)
3.142m of RG213 coaxial cable (makes the 1m diameter secondary coupling loop)
Cable ties to secure the loop to the wooden cross
A variable tuning capacitor (no idea what the rating was, just tried some different ones that tuned)
628mm of RG58 coaxial cable (makes the 200mm diameter primary coupling loop)
Length of RG58 coaxial cable from primary loop to receiver

After construction following this video for guidance (https://www.youtube.com/watch?v=I3JkOxrT9QY&t=236s) I set the loop just 500mm above the ground, connected my AirspyHF+ receiver and tuned the capacitor until I saw the noise floor rise on the 8MHz airband (around 8900kHz).

Plenty of signals including DX from Canada, Philippines, Tokyo etc. so I think that some further engineering using some copper or aluminium tubing and a purpose built capacitor/amplifier will yield some good results and allow me to drop my ugly wire antennas that perhaps don't do justice to the neighbourhood look!

All in all a cheap and very effective antenna for HF monitoring that costs almost nothing and tidies up my property!

 

[majoco]

Very good, Mike. I see there are a couple of youtubes about using a simple amplifier by W7IUN (the amp, not the youtube!) and another that says he lowers the SWR by flattening the pickup loop against the main loop a bit - not that SWR matters much on reception but it may increase the signal a smidgeon.

Your 'increase in local noise' - I too have had about a 10dB increase in noise in a band from about 8MHz up to right on 12MHz - the drop-off at 12MHz is very sharp so it's definitely man-made. I'm wondering if the recent installs around the town with fibre has anything to do with it - I have a telephone exchange about 25metres away and I know that sticks out some strange signals - a big spike on the ISR frequency of 13.560kHz for instance!

NZ$20! You could have made it for free - I have quite a few variable caps in the bottomless junk box!

 

[invergordon]

G'day Martin,

I have had new neighbours in the last year or so and alos the council have replaced the sodium street lights with LED's so I'm not sure who the culprit(s) are. The end game is that I must adapt to my surroundings and deal with it, hence the loop.

I recall our meeting a few years ago where you convinced me a wire antenna could be as effective as my old Wellbrook Loop and you were right but conditions have now changed forcing a review of a more suitable antenna design.

If only I'd know what you had in your junk box!

The only draw back of this particular antenna I built is I have to manually tune the loop for any band change so maybe in spring I will investigate a solution for that.

 

[ka3jjz]

It might be wise to keep at least one of your wire antennas up and active, Mike. It's hard to predict, but there may be times when the loop won't get a signal cleanly (remember, any time we talk about a loop, we're talking S/N ratio, not pure signal strength alone), but the wire does. Propagation can be fickle at any time, so having 2 dissimilar antennas can be an advantage

Mike

 

[majoco]

Hi Mike - one thing that wasn't very clear in the video - is the variable capacitor connected to the outer of the large loop - is it would be if you were using a copper pipe - or is it connected to the inner making a screened loop?

Also he didn't show how the coupling loop is joined - I know (?) but many might not, that the core of the coax is soldered to the braid at the junction - anyone tried two turns? Some youtubes show that flattening the loop at the point of contact reduces the SWR - but is that frequency dependent?

 

[invergordon]

I have the capacitor connected to just the coax outer braid which seem to work OK. I'm not sure there's any mileage in trying the centre conductor. My plan is to use some aluminium flat blade times two to increase the inductance in my version two sometime in spring.

Fair dues, I did have to Google how to connect the primary loop but like most things I do enjoy researching and then figuring out how I would implement my version.

 

[majoco]

Perhaps the youtube poster just happened to have a bit of RG8 with PL259's on the ends that he used. Would add a bit to the cost of the primary loop but does make a nice neat job.
I made trial loop from the remnants of my supposed VLF/LF loop which was quite good - a single turn of power cable around the outside of a square 700mm a side with the coupling loop about 500mm circumference. tuned from about 4 to 16Mhz surprising with a capacitor 30 to 350pF- but not much signal indoors in Sundays rain! I've got some RG8 here, I'll have another go later outdoors.

There's a good youtube where a girl made a double loop from copper pipe for the 160 and 80 metre ham bands - she appeared to have the use of a rather extensive workshop though!