Re: One-off special...Indoor HF Active Loop antenna

Chris Moulding

Hi Graham,

I've had a few suggestions already about supplying your own supply so I might be persuaded to make a few more.

Whenever I mention VLF in the forum or on the Facebook page I'm always surprised at the amount of interest in it and the number of people who have had a professional interest in it.


Chris G4HYG

Chris Moulding
Cross Country Wireless
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mob/workshop: +44(0)7752 391908
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On 26/09/18 21:56, Graham J Smith G4NMD wrote:
But.... if one could supply ones own non-noisy supply.....
I had a long association with Anthorn, Inskip, Rugby and acrimony
Graham NMD
On 26 Sep 2018, at 21:12, Chris Moulding < <>> wrote:

Sometimes we are asked to do one-off versions of our products for special applications, very often it helps push the development of our products into new areas.

Recently I was asked by a short wave listener who only lives a few miles from us if we could build an indoor version of our HF Active Loop antenna. It's not possible for him to erect outdoor antennas but he still wants an antenna that can hear DX and non directional beacons around 300 kHz.

It's an interesting challenge on two fronts. First it has too look good for indoor use so two separate boxes and ethernet cable connection were out and it has to have some form of flat mount to fit on a table. All the amplifier and power filtering was fitted into what would usually be the head unit and I sourced a rather nice oak plinth usually used for making trophies as a base.

The second challenge was even tougher. The standard loop antenna we make at present rolls off at 500 kHz. I used a revised circuit made for another "special" loop antenna but changed the biasing arrangement so that it would work better at low frequencies. I overdid it a bit as I can now hear the VLF submarine transmission from the Anthorn radio station in north west England on 22.1 kHz with the loop. To allow it to be used indoors I added a common mode filter to the antenna input to stop the loop acting as an electric field probe picking up RF noise from the house electrical system. I adder extra common mode filters to the DC input and RF output to further isolate the antenna from noise.

Testing it on HF shows that it will receive DX as ordered. Yesterday evening I was doing comparison tests with a Terminated Inverted U antenna (the lowest noise antenna here so far) on the 80m amateur band and the loop out-performed it even though it was mounted indoors next to two computers.

The improved loop amplifier design is now going to be added to the existing HF Active Loop antenna extending the frequency coverage from 20 kHz to 70 MHz with a dipole loop pattern and multilobe pattern up to 148 MHz.

Unfortunately the indoor version is likely to remain a one-off as I can no longer source a RF quiet linear 12V power supply due to EU regulations outlawing them replacing them with switch mode power supplies in the quest for power efficiency. Every switch mode power supply I've tried generates too much noise even with the common mode filters added to the antenna amplifier. I'm supplying the last of my stock of linear power supplies with it.

Here are some photos of the antenna taken today before it's delivered to the customer tomorrow.




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