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.

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.

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

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.

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?]

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.

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.

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.

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

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.

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.

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/

Re: My first frame aerial
 

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.

Re: My first frame aerial
 

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? :shrug:

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.

Re: My first frame aerial
 

Mount it in one of those vices with the lockable ball joint base?

Re: My first frame aerial
 

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! ;D
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. :idea:

Al.

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.

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".

Re: My first frame aerial
 

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.

Re: My first frame aerial
 

I'm also wondering if they could be used on a transmitter. If they perform on transmit as they seem to on rx it would be very impressive. Again though matching might be a problem.

Re: My first frame aerial
 

Small loops actually make poor antennas for both receiving and transmitting. The difference is that for broadcast reception you are receiving a strong signal in the presence of a lot of noise, so a poor (but very directional) antenna can do a good job. The converse happens for transmission - now the noise is at the other end so the weakness of the small loop becomes apparent. This gives the appearance that antenna reciprocity has failed, but it hasn't.

Re: My first frame aerial
 

Be that as it may the loop is giving me clear reception of stations that I want and rejecting all the noise, hash and adjacent channel interference that a 60 feet long wire gives. The loop can also be spun around 360 degrees in seconds.

Of course if I put filters in the long wire to supress the hash (can it be done?) and bandwidth controls on the receiver to try and stop the adjacent channel intereference and found some way to make the 60 foot long wire go in the direction of my choice I might achieve the same thing as the loop seems to be doing.

You can't do much with a long wire either, just put it up and that's that. The loop is much more fun to play with :)

Re: My first frame aerial
 

Two good Qs. I don't have the experience nor in-depth knowledge to be totally prescriptive about all that, but a moments consideration of 'the basics' indicates the following . . .

With the assumption that a frame aerial has a coupling winding, where the tuning cap. is wired across the main winding, you will have a feeder from that coupling loop to the radio. Since the main winding is a balanced-to-ground affair, (it's important to preserve that) you need a balanced transmission line to connect from the coupling winding to the radio, which will need a balanced aerial input. (Failing that a balun will be needed *). That line will be low-Z, so the tuning cap. must be across the main loop (which will be high-Z) - thus remote from the radio.
As for it being outside, obvious considerations apply. Site it away from earthed objects - especially metallic things - and ensure that the loop - wire, woodwork and tuning cap. - are well protected from harsh weather, incl. strong winds.

* Once, I owned an Eddystone 730/4 comms. radio. That did not have provision for a balanced aerial. However, I was able to re-engineer it so that the coupling windings on the aerial input cols had each earth end lifted up from ground and taken to a separate terminal, thus providing a balanced input. OTOH, the Eddystone 888A which I also owned already had provision for a balanced aerial.
The point is this: if your intended set doesn't have a balanced aerial input, before getting involved with baluns and such like, see if you can modify the proposed radio. A domestic set should be much easier to modify than a six-range comms. radio like the 730/4.

Al.

Re: My first frame aerial
 

Maybe I'm just lucky but I'm feeding the coupling coils output straight into the A and E sockets of an A22 (via 15 inches of twisted flex) and it's working fine. I'll try adding a balun when I get around to it and see what difference it makes or if there is any improvement.

Re: My first frame aerial
 

ISTR that the essence of maintaining a balanced (to earth) arrangement is that by so doing you maximize the depth of the null. The polar diagram for max. signal may also become asymmetrical if the balance is upset.
And, just for what it's worth, I recall that many years ago, I discovered that common twin flex (such as what you are using) has a Zo in the vicinity of 75Ω.

Al.

Re: My first frame aerial
 

Would it really need a balanced input, Al? Connecting the coupling winding to an unbalanced input would simply mean the coupling winding had a ground reference, but as there is no connection to the main tuned winding (other than by a small stray capacity) it shouldn't upset things as far as I can see. The feeder could be coax.

Re: My first frame aerial
 

Thanks for the input all. IÂ’ve just realised that I took a JVC hifi with one of these modern mini loop a.m. loops on it to the tip a couple of weeks back. No doubt it would have had parts for a balum in it. I will be using coax as soon as I can get to Maplins to buy the fittings. How will this affect the need for the balun?

Re: My first frame aerial
 

I suspect that a long length of any low impedance feeder between the loop's coupling loop and the set without some stepdown/stepup transformer at each end will not work brilliantly. A traditional aerial/earth input will be rather higher impedance.

300R ribbon feeder might do quite well....... perhaps with a 1:1 TX at the receiver end to keep it balanced.

I've got a reel of it somewhere, hmmmmm.....

Re: My first frame aerial
 

Thanks. The original plans show coax connected directly across the coupling coil so presumably any matching must be done at the radio end. Can I build the balun myself or do I buy it? A quick look on the internet shows they are band specific with not many mw
ones and also somewhat pricey.

Re: My first frame aerial
 

Strictly speaking it's a n un-un you need as the coax is unbalanced and so is the radio's input. You could try it without at first and if it works to your satisfaction the job's done. If the gain seems a bit low, then a step-up transformer wound onto a ferrite toroid will improve the match.

Re: My first frame aerial
 

The loop rejects local noise/hash because it is less sensitive to longitudinal electric fields than it is to transverse electromagnetic fields. A "long wire" is more sensitive to longitudinal electric fields than it is to transverse electromagnetic fields. This gives the loop an advantage on reception, but not transmission. The loop has narrower bandwidth than a "long wire" and so can reject adjacent channel interference, at the cost of having to retune. This is no advantage for transmission, unless the transmitter is faulty.
You cannot filter away hash if it is at the same frequency as the wanted signal.

I am not knocking loops. They can be very useful for reception under poor conditions. I am just pointing out that their advantages for reception are not because they are good at receiving wanted signals, but because they are quite bad at receiving unwanted signals (local noise, off-channel, other directions). This means that their usefulness for reception does not translate into usefulness for transmission.

Re: My first frame aerial
 

Type of feeder with a frame / loop aerial.
Earlier, I advocated my understanding that for the optimum results with that type of aerial, a balanced feeder was the best (but not the only) choice.

My personal thinking about that goes along these lines.

1. The loop / frame aerial with its tuning capacitor, but not connected to a feeder, is a balanced aerial: it has no connection to earth / ground, etc.*
2. In that sense, as an aerial, it is similar to the classic centre-fed half-wave dipole, without a feeder attached.
3. It is widely accepted - and with good reasons - that a dipole aerial should be fed with a balanced transmission line for optimum results.
4. A consideration of 1, 2 and 3 above strongly suggest therefore that a balanced feeder should likewise be used with a frame / loop aerial for same reasons.

*Strictly speaking, that is not exactly true, of course.
1. There will be some capacitive coupling from each lower part of sides of the frame aerial to ground. And that capacitive coupling will be less at the top of the side windings. However, if the frame is carefully constructed to be physically symmetrical, left and right sides, those capacitances will be equal, thus the balance to earth / ground will be maintained.
2. A typical ex-broadcast receiver tuning capacitor is not a balanced variable capacitor: it is not constructed to be physically symmetrical, although I would expect that lack would have only a very minor 'unbalancing' effect.
However, that does not hold when that capacitor is adjusted (for obvious reasons). I would expect 'Hand-effects' to disturb the optimum settings for elevation and azimuth. But how much? I wouldn't like to predict! ;)

I have added an attachment which is of some relevance and interest. I believe I scanned it from an old PW magazine which I was given a long time ago. Note that this author specifies 300Ω balanced line.

Just a few thoughts: my attempts to develop this fascinating thread.

Al.

Re: My first frame aerial
 

Yes, the tuning capacitor though has to stay with the antenna. But the impedance across the whole loop is too high to successfully pipe it anywhere without capacitive effects upsetting the tuning

If you tap into just one turn at the earthy end (coax braid connected to it and the V/C frame) the impedance and loading effects are low and you can lower it further with a matching transformer (as I do for my transmitting loops) then you can pipe the signal down standard 50 or 75R coaxial cable and transform it up at the other end to suit the radio.

One other trick that could work, if the match wasn't critical just for receiving, since a single turn of the loop has an impedance around a few hundred ohms or more, you could simply use vintage TV 300 ohm ribbon cable (if you could get it) connected to a turn of the loop, feed that to the radio with a 1:6 ratio step up broadband transformer at the radio end terminated with about a 10k resistor at the radio end (or similar) would work. You could probably get away with 5 or 10 meters of this cable as the losses are fairly low.

Re: My first frame aerial
 

Argus - surely any direct connection of a feeder to 'one side' of the loop would ruin the balance, spoil the null and bring about the 'antenna effect' in which the loop begins to act as just a random wire?
I still say a single-turn coupling loop (floating) fed with coax should be fine. A step-up ferrite transformer at the receiver may be necessary if the radio has a high input impedance.

Re: My first frame aerial
 

I'd feed the coupling-loop down a length of 50/75-Ohm coax, with the outer going to the earth-terminal on the radio itself.

But that's because I'm conditioned to using radios designed for 50/75-Ohm antenna feeds.

Interposing obscure baluns etc. in the signal path can only result in signal attenuation - which means the radio's AGC will wind the gain up, so bringing background noise/RFI up as well!

Contextually, I'm wondering if, for MW reception, there could be a place for something like the noise-blanking system used in US WWII aircraft where they phased the signal from the main antenna against the signal from a 'noise antenna' that was deliberately sited in amongst the spark-plugs of the Wright "Cyclone" 24-cylinder radial engine...

Re: My first frame aerial
 

That's exactly what I do. I have a homebrew phasing unit which takes the input from the main antenna (a loop) and a short wire antenna and mixes them together with adjustable amplitude and phase. The loop doesn't pick up much noise, but there are a couple of local SMPS and plasma tellies that barge their way in, and the phaser can work wonders on these if I put my anorak on and get twiddling.

Re: My first frame aerial
 

It is a good question.

I have done much more work with loops as transmitting devices for pantry transmitters than receiving loops for vintage radios. In fact I do not have any receiving loops, only transmitting ones.

When the loop is in receiving mode though, it is sitting in the far field from the radio station's transmitter. At that point the ratio of electric to magnetic field has settled out at 377 ohms or 120pi Ohms. The loop responds primarily to magnetic part of the field. Therefore, in theory, if you grounded any part of the loop electrostatically, one end, its center or near one end, it wouldn't affect the function of the loop as a magnetic receiving antenna. As an electric field receiving antenna, the loop as pointed out, is very poor.

In the case of a loop for a transmitting antenna, I'm confident the magnetic radiation pattern emerging from the edges of the loop, in the plane of the loop, is unaffected by connecting one end of the loop to ground and the coaxial sheaths of cables and other equipments. This is because it it merely an electrostatic connection and doesn't alter the magnetic fields. In the near field, the magnetic component of the loop's output is high and the electric field very low.

When it comes to extracting energy from a loop, or injecting it for transmitting, I think alterations in the weak electric field by unbalancing part of it would probably not be noticeable, that is if you were receiving the signal with a transistor radio with a ferrite rod.

Simply the lower loop turn acts as a coupling coil into the main loop, and if the impedance is transformed correctly, the current in the coupling turn is maximized at the operating frequency. It is a case of "inductive link coupling" into the main resonant loop, so you don't really regard the coupling turn as part of the "main tank circuit" (you could isolate it if you wish as a separate coil one turn coupling coil and not a tap on the main resonant winding) and ideally in this coupling circuit, at resonance, the reactive elements cancel and maximum energy is transferred to the loop and when those conditions are correct it also matches the line impedance.

For my transmitting loops, I have arranged all of them with 50 ohm input impedances. That way I can use my 50 Ohm coax cables, use 50R RF power meters for forward & reflected power meters, SWR meters , 50R dummy loads to check my transmitters, etc and have the loop more remote from the transmitter if I want.

Re: My first frame aerial
 

What is the input impedance of a typical new "hifi"- the sort that comes with a small external loop aerial?

John

Re: My first frame aerial
 

I have sourced some torroids from Ed_Dinning. Can anyone let me know how many windings and turns to wrap around it and what connections go where?

Thanks