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Old 18th Aug 2022, 11:21 am   #1
E93AFAN
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Default Aerial Matching

I have a question to which I am sure everyone else here knows the answer, however despite spending hours combing through back copies of RC and PW I have yet to clarify.

I have built a a number of superhet receivers and a preselector (see example of a Rayer set attached using Wearite coils) and I'd like to try a loop aerial as interference is quite dire where I live. But all the loop designs I can find seem to expect a 50 ohm aerial input. I'm pretty sure the input to my receivers is not 50 ohms (lets honest I have no idea what is is ), so my question is can I match a loop aerial to a receiver without a 50 ohm input and if so how?

I was thinking I might need a balun, but having never made one before what should this look like and realistically can I really make one, but this solution (I assume) will create significant signal losses.

Please be gentle, whilst I have successfully built a wide range of kit over the last 65 years my technical knowledge remains quite poor as my grasp of maths, based on a 1950s education, is totally inadequate. However, I am very practical so can follow "how to" instructions pretty well.

Many thanks as always
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Old 18th Aug 2022, 12:29 pm   #2
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Default Re: Aerial Matching

On your schematics L1 and L2 are the antenna coils.
The windings at 1 and 2 are the modulator coils, the ones that resonate with the variable capacitor at the received frequency.
Windings 3 and 4 are the coupling coils for the antenna. Depending on the number of turns of those, those are already low impedance inputs.
Just by running a few simple calculations, if the coupling coil (3, 4) is about 1:50 of the main coil (1, 2) a 50 Ohm source would be fine as input.
Assuming a modulator coil (1, 2) of 100 turns on MW a coupling coil of 2 turns would be adequate.
Probably the simplest would be to add that two turns by yourself, and not modify the original windings.
Do experiment with the number of turns!

Hope this helps, Peter
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Old 18th Aug 2022, 12:48 pm   #3
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Default Re: Aerial Matching

I'm just in the process of installing a Wellbrook loop aerial. It uses 50 ohm cable and is specified as having an output impedance of 50 ohms. Whether that's the OPI of the amplifier or the supplied antenna interface box, I'm not sure. I'm knocking up a 'converter' box to take a BNC lead from the interface unit and wire it internally to two outgoing 'banana' sockets to connect to my vintage radios, in effect connecting the centre core to the aerial input socket, and the braided outer to the earth socket. Rightly or wrongly I haven't concerned myself with impedances. What do other users of 'Wellbrook type' aerial do to connect to vintage radios? What is the input impedance of a vintage radio? Does it really matter?
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Old 18th Aug 2022, 12:57 pm   #4
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Default Re: Aerial Matching

Be careful because the word 'matching' has a very specific meaning, although it's used loosely all over the place.

Matching really means making two things which will connect together exactly equal in impedance. There is a theorem which says that this will take the maximum power that is theoretically available from a source.

The theorem rather conveniently forgets to mention that you might not want to do this under many circumstances. This keeps the theorem simple, but makes life more complicated and confuses people.

For receivers, their input impedance is usually quite a lot higher than tuned antennae or 50 Ohm coax presents. This seems less than optimum, but there are other factors at work. Most active devices give their best noise performance when the presented impedance is something other than a perfect match. Also receiver input impedances wander around as you tune around. There isn't any indication when the match is right if you have an antenna tuner. Peaking signal is OK if you have an S meter, but if you work by ear, the AGC system will act to reduce changes. Judging by ear for signal to noise ratio is the best you can do... and it's good enough for the purpose.

Transmitters are much more picky about the impedance your antenna system presents to them. Get it wrong and you can have excessive currents, voltages or powers leading to damage. However, this is rarely a match situation, because although your transmitter says 50 Ohms, it means "Please connect me to 50 Ohms" but doesn't mean "I am 50 Ohms" The idea is that if the antenna matches the transmission line (proper meaning of match, here) then there is no reflection, and so the mismatch between transmitter and line doesn't matter.

So cutting to the chase:

For a simple receiver and a general sort of antenna, use an antenna tuning unit. You don't need to pay the costs of big wide-spaced variable capacitors and big inductors needed for high power transmitting. You don't need to do precision matching, but bringing an antenna onto resonance is a big help. The ability to transform its impedance up or down a bit helps a bit and you can get better signal to noise as a result but don't sweat over it it's one of those diminishing returns things as you get closer.

A preselector IS useful on a receiver especially if there are big things around which risk overload. It takes some serious RF design and compromises to work well in big signal and lots of noise environments without the front-end protection of a preselector.

Loops are good, balanced systems are good too. If you're using coax, then baluns pay for themselves.

David
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Old 18th Aug 2022, 1:10 pm   #5
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Default Re: Aerial Matching

So David, with my Wellbrook loop, what spec balun would I need to get to hook up to a vintage domestic valve set? presumably 50 ohms unbalanced to 'something' ?
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Old 18th Aug 2022, 1:15 pm   #6
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Default Re: Aerial Matching

Quote:
Originally Posted by E93AFAN View Post
I'm pretty sure the input to my receivers is not 50 ohms (lets honest I have no idea what is is
400 Ohms or thereabout was a common antenna input impedance for SW RF coils in valve receivers.

EDIT: Post crossed.

Lawrence.
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Old 18th Aug 2022, 1:19 pm   #7
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Default Re: Aerial Matching

Broadcast radios are usually designed to have an input impedance of around 400 Ohms; professional / communication receivers are usually intended to work with a 50 Ohm antenna.

In practice you can get away with a hell of a mismatched connection - unless you are looking to work sub-microvolt DX signals then I wouldn't fret about a bit of a mismatched situation.
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Old 18th Aug 2022, 1:25 pm   #8
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Default Re: Aerial Matching

As David indicates, the 50 ohm output impedance of the Wellbrook loop aerial amplifier is low compared with the typical input impedance of a MW/LW/SW radio. That provides a good match for optimum signal voltage transfer, which is generally what’s needed. So just connect the coax braid to receiver chassis and coax inner to receiver aerial input terminal.

I find that the Wellbrook loop is quite happy feeding a number of receivers in parallel. Mine is currently feeding an AR88 and a PCR2, and sometimes a Pye 39J/H.

Martin
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Old 18th Aug 2022, 1:29 pm   #9
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Default Re: Aerial Matching

Thanks gentlemen for the replies re my post, and I hope I have not hijacked the OP's original questions, although it seems there's a fair amount of useful information for everyone. Thanks.
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Old 18th Aug 2022, 2:02 pm   #10
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Default Re: Aerial Matching

One of the advantages of the big communications receivers, certainly the Racal RA17, and probably the Murphy B40 and RCA AR88, are quite picky about aerial matching, necessary to get down to sub-uV sensitivity.

Domestic radios are not that fussed, and needn't be for that matter.

Craig
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Old 18th Aug 2022, 2:16 pm   #11
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Default Re: Aerial Matching

Quote:
Originally Posted by Craig Sawyers View Post
Domestic radios are not that fussed, and needn't be for that matter.
So why in most alignment instructions did they specify the use of a standard dummy antenna ?

Lawrence.
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Old 18th Aug 2022, 2:46 pm   #12
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Thumbs up Re: Aerial Matching

Thank you very much for your replies, I am in a better place already.

I was seriously thinking of buying a Wellbrook, its a lot of money to spend when you are a pensioner, (or for most on modest means in fact) so I wanted to make very sure I wasn't going to be buy something that would just not work.

Thank you Hartley118, very reassuring and if anyone else has experience of using a Wellbrook with the kind of receivers I have (all valve based, all 1940/50/60 designs and none specifically meant for 50 ohms input) then I would be very grateful to hear from them.

I'll say it again what cracking support, rapid response and yes a great bunch of people you all are, thank you so much and I really wish I had found this forum before.
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Old 18th Aug 2022, 3:53 pm   #13
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Default Re: Aerial Matching

Quote:
Originally Posted by ms660 View Post
So why in most alignment instructions did they specify the use of a standard dummy antenna ?
It depends on the coupling of the antenna into the input tuned circuit. A capacitive or inductive antenna impedance can put the tuned circuit off of the right frequency and thereby lose a lot of signal.

Car radios are probably the touchiest things on the go. The antenna trimmer can have a big effect.

AR88s have quite high Q input tuned stages, and the antenna trimmer on the front panel shows a sharpish peak.

David
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Old 19th Aug 2022, 11:15 am   #14
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Default Re: Aerial Matching

Quote:
Originally Posted by Radio Wrangler View Post
It depends on the coupling of the antenna into the input tuned circuit.
In domestic receivers back then the broadcast band usually used a high inductance primary winding or shunt capacitance for antenna coupling.

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Craig Sawyers View Post
Domestic radios are not that fussed, and needn't be for that matter.
So why in most alignment instructions did they specify the use of a standard dummy antenna ?

Lawrence.
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Old 19th Aug 2022, 2:23 pm   #15
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Default Re: Aerial Matching

I've spoken to Wellbrook and the very helpful man there said that it probably would'nt matter too much to make a direct connection to vintage sets at the lower frequencies that they operate at on LW and MW. However, he is very kindly making me a balun with a turns ratio of 1 to 3 that apparently matches 50 ohms to 400 ohms. So I will include this in my diecast box connected to wander plug sockets for vintage radio use. Otherwise if I ever have to hook up to a 50 ohms receiver I just disconnect the box and use the 50 ohms BNC lead direct to the set.
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Old 19th Aug 2022, 2:41 pm   #16
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Default Re: Aerial Matching

Sounds like a cunning plan!! Best of both worlds..
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Old 20th Aug 2022, 12:44 am   #17
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Default Re: Aerial Matching

Quote:
Originally Posted by E93AFAN View Post
I was seriously thinking of buying a Wellbrook, its a lot of money to spend when you are a pensioner,
Do you know that you can build a clone of the original Wellbrook for very little money?
The DIY Wellbrook is known as the Wellgood; Mr Google will find it for you (George Smart's website) and there are previous threads on this forum. A PCB is available for the amplifier (~£10 ?) which uses just 2 transistors. The loop can be made very cheaply using something like plastic underground water piping.

I have one and I'm very happy with it (it reduces electronic noise interference very greatly). I don't have any vintage domestic radios (I've used it with my HRO and an old Realistic DX160 and some modern radios).

B
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Old 22nd Aug 2022, 3:16 am   #18
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Default Re: Aerial Matching

Quote:
Originally Posted by stevehertz View Post
I've spoken to Wellbrook and the very helpful man there said that it probably would'nt matter too much to make a direct connection to vintage sets at the lower frequencies that they operate at on LW and MW. However, he is very kindly making me a balun with a turns ratio of 1 to 3 that apparently matches 50 ohms to 400 ohms. So I will include this in my diecast box connected to wander plug sockets for vintage radio use. Otherwise if I ever have to hook up to a 50 ohms receiver I just disconnect the box and use the 50 ohms BNC lead direct to the set.
I think that a turns ratio of around 3:1 was typical of the transformers used in the antistatic aerial systems of old. That is a 3:1 step down at the aerial end, and a 1:3 step up at the receiver end. Typically they used screened twin of around 75R, but the same ratios would work for coax.

At one time I think that Wellbrook also offered its UMB130 “isolating balun”, with a 9:1 impedance ratio (3:1 turns ratio) primarily for matching longwire aerials (highish impedance) to coaxial cables. The same would work in reverse, as it were, for matching coaxial cable to the highish impedance of domestic AM receivers.

My recollection from a long time ago (and then some) is that domestic receivers vary in behaviour when connected to a low impedance aerial source. But I think that most have loose enough coupling of the aerial input to the first tuned circuit that it does not much matter. One or two – e.g. some Bush models – have two aerial inputs, one more tightly coupled for maximum sensitivity, and the other more loosely coupled for maximum selectivity, i.e. minimum detuning of the input circuit. Something else, I think that communal aerial system drops usually had a resistor of around 1k between the tap and the aerial socket, probably in part for isolation but maybe also to ensure that domestic receivers were looking back into a “familiar” impedance, just in case some were fussy on that front.

Another recollection, not rigorously verified, is that when the aerial inputs of domestic receivers were looking back into a closed circuit, such as a the secondary of a balun (or unun) transformer, then interference pickup was reduced somewhat , regardless of what was kind of aerial was in use at the other end of the coax. At the time the rationale I imputed (not necessarily correct) was that interference voltage picked up on the earth path (be it conductive or capacitive) from the bottom of the aerial input coil to wherever was the nearest earth potential was no longer effectively series with the aerial voltage, so had reduced effect.


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Old 22nd Aug 2022, 5:46 am   #19
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Default Re: Aerial Matching

Most antenna impedances vary with frequency. In the case of long wires and beams, this can be quite sharp. The Wellbrook loop and its derivatives is an active antenna with a buffer amplifier built into it, so it's impedance presented to the line is kept more stable.

Most radio receivers offer poorly defined impedances to load feedlines. Usual broadcast sets of the period were expected to be used with long wire antennae, usually far short of resonance, so the impedance they were expected to handle was high, and capacitive. To avoid the need for an aerial trimmer control, the Q of input tuned circuits was kept low, even though this further compromised image rejection. Some 'communications' sets like the AR88 were given antenna trimmer controls on the front panel, allowing tighter Q in the input circuits. Also this means a greater voltage magnification factor (itself an old name for Q)

At LW-SW frequencies, the input impedances of valves are high and capacitive. Any upward transformation either by a broadband transformer or by a resonant tank circuit is likely an improvement, especially from a relatively low Z like a coax cable... It's probably best not to think of matching so much as a less-bad mismatch reducing, not eliminating impairments.

To keep things in perspective, we have the evidence of many many years of production of sets which work quite well enough.

It's when we get to VHF and above that we need to fight for every bit of sensitivity and reduction of noise figure that we can get, and we have to make hard compromises between gain-matching and noise-matching.

So for a low-Z antenna, like many resonant ones and active ones (Even 300 Ohms is low in this context) some step-up voltage transformation is beneficial. There are various ways to do it, none perfect.

A tuned circuit can be very good, but it needs to be tuned tracking the rest of the receiver and it'll need a twiddler. There is a relationship which says that the more the (resonant) transformation ratio you shoot for, the narrower the frequency range it works over, hence the need for tracking and trimming.

Ferrite toroidal cores are a big help in making broadband RF transformers. But designing a transformer to work over much frequency range AND much impedance range is still difficult. If your turns ratio and magnetising inductance force you into many turns, stray-C in the larger winding soon brings trouble with spurious resonances, while fewer turns would reduce the magnetising inductance and truncate your coverage at the low frequency end. In the case of the Wellbrook, its amplifier gives it a helpfully well-defined output impedance, so a step-up transformer at the receiver end of the line should work well into a broadcast set or into a classic communications receiver. 3:1 in turns ratio (=9:1 in Z ratio) shouldn't be problematic. Go higher and you'll start to run into frequency range limitations. A resonant transformation ox could be good, acting as a preselector giving the set some protection from overloading signals off frequency, but it'll need tuning along with the set, and if you have an active antenna, its amplifier isn't protected and it gets hit by everything in your vicinity.

Compromises within compromises within compromises.

But precision matching isn't required, things are a long way off perfection, but still can be good enough.

Transmitters are another matter, they have their ways of letting you know if they don't like the Z they are given to drive. Smoke, flames, smells, but mostly money.

David
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Old 10th Sep 2022, 4:25 pm   #20
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Default Re: Aerial Matching

Here's the balun to more accurately 'match' the 50 ohms cable from the Wellbrook loop to a vintage radio that is around 400 ohms. I have't tried it yet due to other commitments.
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