Saturday, December 20, 2008

80m receive loop antenna on a crochet hoop

Here's a very simple but effective receive loop antenna for 80m wound on a 35cm diameter crochet hoop purchased at Spotlight.

LoopOverview.jpg

The tuned part has five turns with a polyvaricon variable capacitor in parallel. There is one turn with a 0.01uF capacitor that goes off to the antenna input socket.

I think the tuning capacitor is about 365pF.

Compared to the radio's whip antenna, signals are stronger and when at the best alignment have lower noise. The other advantage of loops is the ability to null out noise sources too.



loop circuit.pngWhen I started this, I thought more turns would be needed so I used computer ribbon cable held in place with Velcro straps.

The tuning range is 1860kHz - 4000kHz with the polyvaricon trimmers all the way closed. (I'll add a little more capacitance so I can cover 160m)

This would be a practical antenna for travel applications and could be particularly useful for nulling out local interference.

I was thinking of building an rf preamplifier but I'm finding that the Degen radio is quite sensitive enough.

loopCloseup.jpgThe little control box is a potting box with slots cut for the crochet hoop to slot in to it so the whole thing can stand up.

As I'm mostly going to use it with little portable radios I've just gone for a 3.5mm socket for the RF output.

Below is the simple guts of it.

Not pretty and rather brittle, once again a hot glue gun is my friend.

LoopBeneath.jpg

8 comments:

Anonymous said...

Hi,

A very interesting and enlightening post. Just a couple of queries:

- the schematic sketch shows a *variable* capacitor in series with the pickup (secondary) loop - should it be the *fixed* 0.01uF ceramic shown in the photograph?

- what's the purpose of the 0.01uF cap, anyway?

- the polyvaricon variable capacitors from Dick Smith, etc., have a range of only 60-160pf, although yours may be from a different source. But in the video, you're using an air variable which seems to have a much larger range. How was the tuning range with the polyvaricon?

- your secondary (pickup) winding is fairly tightly coupled to the main (tuned) loop - well, by comparison to a vastly smaller, separate loop placed inside the main loop, which seems to be popular. But it seems that you are obtaining perfectly adequate Q, without the trouble of a separate pickup loop. Easier = better ...

- did you get it onto 160?

- no balun? I see Alan Yates using a ginormous ferrite ring (http://www.vk2zay.net/article/141)for his "HF Receiving Loop."

Thanks again !

Peter B Marks said...

Dear Anonymous,

Yes, you're right a variable capacitor isn't needed in the loop to the receiver. The purpose of that capacitor is to avoid heavily loading the pickup loop.

No, I wasn't able to tune down to 160m.

I saw a large loop by VK2ASU that had switchable turns which allow more coverage, that might be a good next step.

Yes, Alan VK2ZAY is using a bicycle wheel. John VK2ASU is using a Hula Hoop so I'm the little guy here.

w1gfh said...

http://amfone.net/Amforum/index.php?action=dlattach;topic=22919.0;attach=17189;image

Peter B Marks said...

A great looking loop, well done and thanks!

Anonymous said...

I'm planning to make a tv antenna the one I bought (with loop) only picks up 2 bearable channels, the others are completely nothing, do you think i would benefit considerably from a signal amplifier, since i live near one radio station about 3 kilometers from here, thant's the only channel with clear signal.

Anyway Nice Antenna.

Anonymous said...

I'm planning to make a tv antenna the one I bought (with loop) only picks up 2 bearable channels, the others are completely nothing, do you think i would benefit considerably from a signal amplifier, since i live near one radio station about 3 kilometers from here, thant's the only channel with clear signal.

Anyway, Nice Antenna.

Peter Marks said...

This item is about an antenna for a very low frequency, 3.5MHz, a TV antenna is meant for a much higher frequency such as 70MHz.

I have seen indoor TV antennas with a small loop but I'm not sure if it really works of if it's for show. My feeling is that the best improvement for you would be to install an external TV antenna up higher and pointing to the stations you are trying to receive.

Emi Sam said...

There are many store brought antennas that have the capacity to pick up just a few channels. You will just need to do some adjustments or create a custom made one like this for maximum signal strength.