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Old 8th Dec 2007, 3:12 pm   #1
Al (astral highway)
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Default How to make a cheap but effective loop aerial from DIY store parts

I've just build my first, tuneable shortwave antenna and it's a great success. I'm a complete SWL novice, I'm not talking from a position of expertise in this area.


The main loopis a single 3 metre length of household mains earthing cable (the thicker grade - about 5 mils). I chose this length because Speedfit Barrier tubing comes in 3 metre lengths and this happens to be a perect length for the antenna's main element. (I think you end up with an inductor of roughtly 2.5 uH) Also, it's a perfect fit for a low-inductance thick conductor, which fits snugly inside.

I formed the main loop and then attached the two ends to a small dowel. I strain-relieved the loop to the dowel with heavy cable ties, then made the heavy cable connections to the 560 pF tuning capacitor, (a thing of beauty that certainly didn't come from a hardware shop!)

The coupling loop is about 60 cm of 30A single strand cable.

It is cable-tied to the mast, then soldered to the transmission line. Here, I'm using 75 ohm tv-to-video cable, because that's what I had to hand. It works fine.

The base of the unit is a simple wooden 'cross' shape. The (otherwise top-heavy and wobboy) vertical is maintained by a single length of tension, equalised cable that runs through the mast and secures to the feet via clove-hitches and a loop respectively.


As I write this, I'm tuned into Radio Rossii at 7.312 Mhz on a LOWE SRX 300 that got yesterday.

Untuned, the aerial receives a signal of strength 2-3. By tuning the aerial capacitor, I reduce noise and get an obvious sharp tuning peak and a signal strength increase from 9-max.

I'm particularly amazed because the whole rig is only a metre away from my laptop and USB port, which radiates a humungous amount of RFI, none of which is noticeable now.


So there we go. Apart from the tuning cap, which was erm, about £15, the rest of the unit cost less than a fiver, and produces excellent results.


Benefits:

tuneable - great selectivity
noise reductdion
directional
cheap
easy-to-build
portable


I'd welcome any comments or observations!
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Old 8th Dec 2007, 4:37 pm   #2
MichaelR
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

well done Al,

I built a magnetic transmitting loop about 15 years ago from normal central heating copper Tubing ( formed as one piece into a circle ) . I metre diameter with a vacuum capacitor it tuned 14mhz to 30mhz. I used a stepper motor to remote tune the capacitor and used it for about 5 years it being on a Pole lifted about 6 foot off the ground.

For transmitting it compared favourably with a multiband trapped dipole at a height of 30 foot although I would not have said that it consitently beat it on performance.

You should be able to get good directional performance by rotating the loop about its vertical plane

mike
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Old 8th Dec 2007, 5:06 pm   #3
Al (astral highway)
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

Thanks Mike! I'm delighted that the aerial seems to perform very well at the forty metre waveband.

Next I want to pin down exactly which wavebands this combo tunes the best. I'll be waiting till it's a bit later into the night , but great to hear Radio China loud and clear last night.

What I really like is the way the aerial tuning kills the noise and produces enormous gain (I'm guessing that the receiver's signal indicator is a logarithmic scale, so represents a doubling of power between divisions? Is that likely?
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Old 8th Dec 2007, 5:24 pm   #4
MichaelR
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

Hi Al,

I used commercial traps but cut and assembled the dipole myself, at that time I had access to a swept Scalar network analyser hence could easily measure a swept return loss of the aerial.

Just as you are aware with the mag loop that you have made, keeping the losses as low as possible is important to maintain a high Q ( this is what is giving you the amazing reduction in noise ).

The lower you go in frequency however (to meet low swr transmit requirements) the high Q can be such that the response gets very narrow and for broadcast AM listening may be too narrow.SSB/CW needs far less bandwidth.

Mike
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Old 8th Dec 2007, 5:34 pm   #5
Al (astral highway)
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

Quote:
Originally Posted by MichaelR View Post
Hi Al,


The lower you go in frequency however (to meet low swr transmit requirements) the high Q can be such that the response gets very narrow and for broadcast AM listening may be too narrow.SSB/CW needs far less bandwidth.

Mike
I see here three 'modes' on the receiver - I'm assuming modulation modes. They are AM (currently tuned) USB and LSB. I don't want to go off-topic but what are these used for?
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Old 8th Dec 2007, 5:52 pm   #6
Patrick Dixon
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

Upper Side Band & Lower Side Band?
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Old 8th Dec 2007, 6:12 pm   #7
murphyv310
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Default Re: How to make a cheap but effective loop aerial from DIY store parts

Well Done!
I made a similar loop some years ago using a "hula hoop" it was excellent and used it for MW DX even receiving US MW stations.
I bought a Wellbrook 1640P loop last year and its truly amazing, good from long wave right up to VHF FM and its NOT tuneable. I used it in the summer and received good band 1 TV DX, I dont work for Wellbrook but would reccommend them to anyone.
Best hobby in the world with Medium Wave really interesting, try getting up early in the morning before sunrise and tune a few tens of khz above 1600khz and listen for US stations, you will be surprised!
Trevor
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