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Old 12th Feb 2018, 5:13 pm   #1
Boom
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Default My first frame aerial

With immense thanks to David Fernseh for his advice and encouragement I have just for the first time built a frame aerial. The design I got off the 'net was a 25" wide one with 15 turns of wire which needed to be threaded through 69 holes!

I was somewhat sceptical of what a coil of wire could add to a receiver but indoors it brings Caroline up from a hiss in the background to a good useable signal. The mains hash seems to almost vanish.

At some time in the future I will make up a standard one metre square aerial but for now I am well happy

I tried to take a photo' to upload but for some reason the windings create almighty moire patterns which destroy the picture.
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Old 12th Feb 2018, 7:31 pm   #2
ukcol
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Default Re: My first frame aerial

A frame aerial can also be useful if you suffer with a local source of (air borne) interference that cannot be dealt with at source. The directional properties of the frame aerial can be used to null out the interference.
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Old 12th Feb 2018, 7:41 pm   #3
G6Tanuki
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Default Re: My first frame aerial

Interesting to hear of your experiments: I may have to try something similar myself, to see if I can improve Caroline reception.

One thing I've never understood about many of the MW frame-antenna designs seen on the Web is that they always seem to space the turns out over some distance, which must surely make them more susceptible to picking up electrostatic noise and also reduce the sharpness of the null. I'm more used to loop-antennas being wound 'tightly' with the turns on top of each-other, and provided with a Faraday screen so they don't pick up the electrostatic component.

See the DF loops fitted to WWII-era aircraft: they invariably shielded the loop.

http://www.rfcafe.com/references/ele...ary-1963-1.jpg
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Old 12th Feb 2018, 8:16 pm   #4
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Default Re: My first frame aerial

The wide spacing of the coils was mentioned on a site I was on yesterday. I think the reason given was to keep the capacity of adjacent coils down so that the tuning cap would cover a wider range. Whether this is the only reason I don't know.

The null on this one is very sharp indeed.
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Old 12th Feb 2018, 8:24 pm   #5
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Default Re: My first frame aerial

Always interesting to see what works for those who've gone before me - I can understand spacing to reduce self-capacity, but equally it occurs to me that a bit of extra self-capacity could be offset by reducing the number of turns on the coil.

My thinking being that self-capacity will be less-lossy than the resistance of the windings (unless you're using Litz?) so reducing the number of turns by a couple to offset the increase in self-capacity might actually improve the "Q" of the result?

[Or have I perhaps opened the Shiraz too early and am using voodoo delusional physics?]
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Old 12th Feb 2018, 9:10 pm   #6
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Default Re: My first frame aerial

I think the way it works is something like a 'standard' tuning cap is 350-500pF and if the coils were close wound and added up to say 200pF that would effectively put the 200pF in series with the tuning capacitor greatly reducing its range. With a wide coil spacing their capacity is almost zilch allowing the tuning cap' to cover the entire range of almost zilch to 350/500 pF.

That's my reading of it anyway. I just checked this one and it's covering the entire medium wave and has a bit left over.

I just added an extra 1 turn coil and it is now feeding the A22. I've never heard it receive so well. It's leaving the longwire standing.
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Old 12th Feb 2018, 9:20 pm   #7
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Default Re: My first frame aerial

The self capacitance is in effect in parallel with the inductor, the higher the value of self capacitance the lower will be the highest frequency tunable with a given parallel connected tuning capacitor.

Lawrence.
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Old 12th Feb 2018, 9:23 pm   #8
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Default Re: My first frame aerial

I built a frame aerial, which I regularly use. Mine is 32 inches square. Has ten turns with one turn pickup. I am not sure offhand what my tuning capacitor is without looking, but something in the hundreds of Pf. It often works better than the long wire, even though a few feet lower (but not always). It is tricky for me to use it's directionality due to it's size in my small room.

I would recommend them for MW.
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Old 12th Feb 2018, 10:32 pm   #9
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Default Re: My first frame aerial

I have a memory of a MF loop aerial down in the darkest cellar of my memory that had a 500pF variable capacitor strapped across the loop, I presume to tune the loop a bit?
My radio knowledge is scrappy so sorry if I got a bit "muxed ip".
This Idea of a simple loop appeals to me as I like to listen to the local station on MW in the garage or shed and anything that helps me get it stronger appeals.

A
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Old 13th Feb 2018, 11:37 am   #10
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Default Re: My first frame aerial

Was it this 40 inch square behemoth? I'm just about to start building it!

Taken from Practical Wireless, November 1966.
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Old 13th Feb 2018, 12:58 pm   #11
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Default Re: My first frame aerial

Construction of a frame aerial. It might just be worth mentioning that apart from the ability to alter its azimuth, the ability to adjust its elevation can also be useful: adjust to suit the downward angle of the arrival of the E-M wave, if that wave is an ionospheric reflection, as opposed to a ground wave. My understanding is that it's the vertical parts of the loop that pick up the signal; ideally, you want zero pick-up on the horizontal sections.

Al.
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Old 13th Feb 2018, 1:56 pm   #12
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Default Re: My first frame aerial

This is the one that I cloned. It is the 4th one down at the bottom of the page.

It is only 25.5" across but goes like a good 'un. The only pain in the trousers is drilling and threading the 69 holes.

http://makearadio.com/loops/
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Old 13th Feb 2018, 2:29 pm   #13
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Default Re: My first frame aerial

Quote:
Originally Posted by Kentode View Post
Was it this 40 inch square behemoth? I'm just about to start building it!

Taken from Practical Wireless, November 1966.
This sort of thing also makes for an excellent transmitting antenna for a pantry transmitter, provided you tap into just one turn of the 7 and appropriately impedance match that into the RF output stage of the transmitter when the loop is resonant at the transmitting frequency.
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Old 13th Feb 2018, 2:41 pm   #14
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Default Re: My first frame aerial

Quote:
Originally Posted by Skywave View Post
adjust to suit the downward angle of the arrival of the E-M wave, if that wave is an ionospheric reflection, as opposed to a ground wave.
You are correct. I have just tried it and there is some skip, obviously not a lot during daylight but the aerial appears to be able to differentiate between skip and ground wave.

Now how do I build a system into the aerial to allow fore and aft tilt?
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Old 13th Feb 2018, 3:05 pm   #15
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Default Re: My first frame aerial

I have used loop antennas for MW sets that I have built over the years. They are loosely based on designs found in your typical battery valve portable set. They are quite small, usually about 12" square and actually replace the first tuning coil in the radio. They work quite well, and are directional which is handy for nulling out interference. I just wind them to give the correct inductance to resonate with the tuning capacitor, and use enamelled wire. The only problem is that you can't vary the inductance easily to adjust the tracking. With a bit of experimentation and removing turns you can get it near enough.
Alan.
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Old 13th Feb 2018, 4:01 pm   #16
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Default Re: My first frame aerial

Quote:
Originally Posted by Boom View Post

Now how do I build a system into the aerial to allow fore and aft tilt?
Mount it in one of those vices with the lockable ball joint base?
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Old 13th Feb 2018, 5:07 pm   #17
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Arrow Re: My first frame aerial

Quote:
Originally Posted by Boom View Post
Now how do I build a system into the aerial to allow fore and aft tilt?
To a considerable extent, that can be determined by your ingenuity and available tools.

The simplest idea that immediately springs to mind is to make a joint in the vertical spindle above the rotating joint, and arrange that new joint to have a hole drilled through it with a suitable length screw and a wing nut. You manually adjust the elevation by trial and error, then lock that elevation setting with the wing nut.

A more ambitious approach would be along these lines. Arrange a horizontal beam, below the frame, so that it rotates with the aerial. You still need a joint as above, but without the locking wing nut. If you run pieces of string from each top corner of the frame down to the extremities of that horizontal beam, it should be possible to contrive some sort of winding mechanism so that when one piece of string is wound up, the other is unwound, thus tilting the frame in a vertical plane. The 'mechanism' may require things like crank handles, springs and a ratchet and pawl. This second idea is simply to inspire your imagination!
Overall, though, that is quite a complicated affair: something between my two ideas could probably be devised. (Elementary on one hand; a bit OTT on the other! )

Let's see what ideas other members suggest.

Al.
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Old 13th Feb 2018, 7:14 pm   #18
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Default Re: My first frame aerial

Could one of these loops be mounted away from the radio, say on the roof of my shed/summer house and a feed led in to the radio a few yards away?

And would the tuning/matching capacitor be ok mounted by the receiver?

I had somewhere in all my junk one of those old PW data wallcharts with details of various aerials plus I think, an ATU that was switchable with a tapped coil as well as a variable capacitor. Wish I could find it again, I've lost loadsa bits and bobs since the house move 18 months ago.

Andy.
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Old 13th Feb 2018, 7:25 pm   #19
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Default Re: My first frame aerial

Loop so far. Once I get the hang of it I'll build a tidier one. The loop is dragging in Caroline fine without a trace of the Spanish rubbish but can't hear Caroline at all on the long wire tonight just the Spanish station.

I've taken the picture next to an A22 to give a perspective of the size. The loop beam is 25.5".
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Old 13th Feb 2018, 7:44 pm   #20
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Default Re: My first frame aerial

Quote:
Originally Posted by bikerhifinut View Post
Could one of these loops be mounted away from the radio, say on the roof of my shed/summer house and a feed led in to the radio a few yards away?

And would the tuning/matching capacitor be ok mounted by the receiver?
I'd be worried that the stray-capaticance between the two wires running from the loop to your variable-capacitor - appearing in parallel with the variable - would compromise the tuning range.

Obvious answer is to use a variable-capacitance diode fitted directly at the loop, then the wire back to your control-point only needs to handle DC.

Searching for "varactor tuned loop" shows plenty of examples.
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