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16th Jul 2022, 12:16 am | #1 |
Triode
Join Date: Nov 2009
Location: Bergen, Norway
Posts: 39
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Setting up an AM antenna
I have usually just used a long copper wire for AM antenna, but I found this video on the subject. Does anyone know what that thing in the middle is? It is a sort of fine tuning of the signal. He calls it a simple variable capasitor and talks in a way that we all are familiar with the finer points of antenna building. Can I just buy any on ebay and make my own DIY attempt?
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16th Jul 2022, 2:09 am | #2 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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Re: Setting up an AM antenna
It is a regular variable capacitor (air-spaced) and he's quoting a range of something like 15 to 420pF for it. There seem to be quite a few on eBay which would do nicely, but the prices are alarmingly high. You don't need to match it exactly for the min/max values, but if you use anything smaller (~365pF max) you may need some extra turns on the loop to get resonance (he quotes the formula for resonance).
There are variable caps with solid dielectric, and if you can find an old radio with one in it, you could save some money. B
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16th Jul 2022, 5:42 am | #3 |
Pentode
Join Date: Jul 2014
Location: Bangalore, India
Posts: 132
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Re: Setting up an AM antenna
It's an AM loop antenna that would give maximum signal strength with the plane of the loop in the direction of the station.
A null would be obtained with the normal to the plane of the loop pointing towards the station. The component, at the centre of the loop, is an air-spaced variable capacitor connected across the loop. Such a capacitor may be salvaged from an old valve radio. 'PVC' variable capacitors, salvaged from old transistor radios, may also be used. 'Antenna' and 'oscillator' sections of the capacitors may be connected in parallel to increase the capacitance. The capacitor is used to tune the loop to resonance at the required frequency. The ferrite antenna of a portable radio may be inductively coupled to the loop antenna, placing the radio in close proximity for improved reception of stations.
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16th Jul 2022, 8:46 am | #4 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,422
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Re: Setting up an AM antenna
Hi.
Make a frame aerial, a meter square will have 8 turns with a single coupling turn to the radio. Tuning cap 500pf. They work well. Here is the one I use that actually works as a crystal set! https://youtu.be/t-NM3tZZgWM
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16th Jul 2022, 9:46 am | #5 |
Hexode
Join Date: Nov 2018
Location: Greater Manchester
Posts: 305
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Re: Setting up an AM antenna
I built one a couple of years ago. Very effective but rather cumbersome.
Lynton
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18th Jul 2022, 11:25 pm | #6 |
Triode
Join Date: Nov 2009
Location: Bergen, Norway
Posts: 39
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Re: Setting up an AM antenna
Thanks for reply all of you. Your home made antennas look nice too. I shall have to order the capacitor, I have no old radio to sacrifice. My working AM radio is a Phillips 50s valve radio with a record player.
I have looked at several loop antenna tutorials. Some make an antenna with the end of the copper wire (from the loops) connecting and are left with two connector plugs at the end of the antenna. Some seem to make the loops separate, and then make the antenna wire that connects to the radio inside the loops. My old radio has one single banana plug for connecting antenna, and I think one for earth. A second type antenna I have found that's suppose to be good is a spiderweb antenna. I assume it works in a similar way, but the copper loops are done flat on a pice of plywood, often pentagon or heptagon shaped. I think most radio broadcast (if I can find any) still are medium wave, but my radio have three settings; long wave, medium wave and short wave. Are there separate anennas for various wavelengths? |