Setting up an AM antenna - UK Vintage Radio Repair and Restoration Discussion Forum

silver ghost · 16th Jul 2022, 12:16 am

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?

Bazz4CQJ · 16th Jul 2022, 2:09 am

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

vu2nan-nandu · 16th Jul 2022, 5:42 am

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.

murphyv310 · 16th Jul 2022, 8:46 am

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

LyntonP · 16th Jul 2022, 9:46 am

I built one a couple of years ago. Very effective but rather cumbersome.
Lynton

silver ghost · 18th Jul 2022, 11:25 pm

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?

16th Jul 2022, 12:16 am	#1
silver ghost Triode Join Date: Nov 2009 Location: Bergen, Norway Posts: 39	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?

16th Jul 2022, 2:09 am	#2
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	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 __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

16th Jul 2022, 5:42 am	#3
vu2nan-nandu Pentode Join Date: Jul 2014 Location: Bangalore, India Posts: 132	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. __________________ https://nandustips.blogspot.com

16th Jul 2022, 8:46 am	#4
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	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 __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

16th Jul 2022, 9:46 am	#5
LyntonP Hexode Join Date: Nov 2018 Location: Greater Manchester Posts: 305	Re: Setting up an AM antenna I built one a couple of years ago. Very effective but rather cumbersome. Lynton Attached Thumbnails __________________ Never buy a mermaid a pair of socks!

18th Jul 2022, 11:25 pm	#6
silver ghost Triode Join Date: Nov 2009 Location: Bergen, Norway Posts: 39	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?