Aerials for old domestic valve sets.

[G8HQP Dave]

That is OK as a starting point for someone who knows nothing about antennas, but I spotted a few errors on the first page.

[ms660]

Originally Posted by G8HQP Dave

That is OK as a starting point for someone who knows nothing about antennas, but I spotted a few errors on the first page.

It was intended as an introduction for beginners, I must admit I haven't studied it in depth.

Which are the errors on the first page?

Lawrence.

[crackle]

G8HQP Dave, I dont understand your statement "Being short is not a problem." Maybe I am mis-reading that.
I know from practice and experience, at the MW & LW frequencies we are talking about here, that the longer an aerial (within reason) the stronger the signal pickup is.

A simple test with a 2 foot wire in the aerial socket of a set, compared to a 10 foot wire will show a distinct difference, whether it is vertical or horizontal. The longer always gives a stronger signal, (both wanted signal and unwanted interference) if you use a 60 foot wire it is stronger still. But moving the wire away from the interference loops around the house, as I have, has greatly reduced the unwanted interference.

In the case of my inverted L aerial with a long and high horizontal part, I would eat my hat if the reception strength did not drop significantly if I replaced the horizontal wire with a rope.

Mike

Mike

[DoubleWound]

I am finding this discussion theoretically and practically fascinating since no matter what aerial I try, RF interference increases in direct proportion to increasing signal strength in my abode.

So today I conducted an experiment with a Marconi T29A which has an internal foil aerial and is connected by three core cable to mains earth.

Here is the test rig:



1. Well away from the house the set was powered by a generator and obviously no mains earth. RF interference was negligible on and off station.

2. Then the set was connected to the mains with the same result.

3. Then the aerial socket was connected to a few metres of wire and signal strength went up but no increase in RF interference.

4. Then I couldn't resist connecting the aerial wire to the metal framed greenhouse and signal strength further increased but no increase in RF interference.

From this I conclude that my local in house interference is not specifically mains borne, either from the two mains conductors or earth conductor, but does not preclude the ring main radiating signals picked up from other sources.

So this type of set will always pick up in house interference via its internal aerial although a well designed external aerial would perhaps help to swamp the unwanted house noise.

Maybe I should just stick to FM sets!

[Mike. Watterson]

Lighting circuits are often the worst, with separate runs of live to the switch. Also above the foil backed plasterboard.

[LyntonP]

Spent a little more time with the 40 inch loop today. Re hashed the feeder and capacitor connections with very good results. Took the aerial and the Trio R600 outside. Tuned around MW for a weak station, rotated the aerial for best reception then tuned the capacitor with surprising results The S meter went up from S3 to + 20 dB. So the loop is working!
Capacitor tuning only varied a little on MW and as previously reported the aerial is very directional. The results on LW not as clean cut. Still a strong clear signal but capacitor tuning not as sharp. I read somewhere that for LW it is advisable to remove one of the aerial loops 6 instead of 7, but didn’t have time to try this. I have looked on the internet for a smaller loop that would be more convenient indoors but the only thing I kind find are several American articles on ferrite or “loopstick” aerials.
Has anyone come across a smaller design that still works?

[Synchrodyne]

Originally Posted by crackle

G8HQP Dave, I dont understand your statement "Being short is not a problem." Maybe I am mis-reading that.
I know from practice and experience, at the MW & LW frequencies we are talking about here, that the longer an aerial (within reason) the stronger the signal pickup is.

I read that as a contextual comment, countering the preceding assertion that short vertical aerials were useless for LW and MW. The latter looks as if it may have been conflation of quarter-wave verticals as used at HF and VHF, with the short verticals used for MW and LW reception. Definitely the longer (taller) the vertical is, the greater is its signal (and interference pickup); field strengths are typically measured in mV/metre after all. However, even a very short vertical can pick up enough signal to be useful within the primary service areas of LW and MW transmitters. At one time, the boundary of the first class service area of an MW transmitter (absent serious co-channel and adjacent channel interference) was considered to be at the 2 mV/m contour, with good reception still possible in electrically quiet rural areas at the 0.5 mV/m contour. At the 2 mV/m contour, a 2 metre vertical will pick up enough signal I think for adequate performance from even modest domestic receivers. Sturley outpointed that as long as the effective height of a short vertical is below around 0.2 wavelength, then its terminal impedance variations are not of magnitude to cause too much upset to the input tuned circuit of receivers, whereas as they approach resonance, there can be significant problems. One could say that the short vertical is a different species to the quarter-wave vertical.

With an L aerial, the horizontal part does not in and of itself pickup much vertically polarized signal, but it increases the effectiveness of the vertical, resulting in more signal. That is nicely explained by Sturley (pp.73-75), with the comment that a horizontal-to-vertical ratio of 1.0 increases the effective height to 0.6h, the nominal being 0.5h.


Cheers,

[Synchrodyne]

Sturley also provided a useful commentary on anti-interference aerials, attached:

Sturley Interference Reducing Aerial Systems.pdf

The commercial versions of this type from the major aerial manufacturers appeared to be named as follows:

Aerialite: Mastatic and Antistatic

Antiference: Exstat

Belling & Lee: Eliminoise (with the vertical rod aerials known as Skyrod)

All offered their systems for use with both vertical rod and L aerials, and all used twin-screened feeders.



The BBC had its own version using a Skyrod and coaxial feeder:



BICC did differently, offering a 60 ft dipole with a coaxial feeder and matching transformers at the aerial and receiver ends.




Cheers,

[Hybrid tellies]

In the mid 1970's we moved from the coastal village of Coverack to Helston. It was a MOD housing estate and all the houses had large gardens.

I installed my HMV 1124 valved radio in the bedroom connected to about 60' long wire aerial across the garden to a fairly short makeshift wooden post at the bottom of the garden with the earth connected to the mains earth. Reception was ok but the lower frequencies including the Long wave, Medium Wave and the lower frequency short wave bands were effected by electrical interference from the surrounding houses, nearby shops and some nearby small local industries.
I remember I was getting a continuous warbling sound in the background of BBC R2 on 1500m and during the evenings interference from television sets was severe with that loud whistle on Long Wave and a continuous hash like noise across much of the Medium and Short Wave Bands.
I was given a slightly taller more solid wooden post and lifting the height of the long wave by about a meter aerial did improve reception with quite a noticeable drop in noise and interference levels and that warbling noise in the background of BBC R2 on 1500m cleared.

But the dramatic improvement in reception, especially the reduction in electrical interference and noise came courtesy of the GPO. They installed a nice earth post in the garden just below my bedroom window. Using this earth instead of the mains earth made a huge difference and as well reducing the local electrical noise and interference but improved the signal pick up especially on the lower frequency bands. Reception of all the BBC stations from Redruth, Start Point and Droitwich was loud and clear with no background noises heard. During the daytime I was now able to listen to the more distant IBA local stations from Plymouth and Swansea as well as pick up RTE from Cork and Manx Radio.
Looking back I think this was an important lesson demonstrating the importance of a good radio earth when using a long wire aerial. I am not sure how this would fare today but I suspect it could still make a big difference.
Sadly where I live now I am unable to install a decent radio earth.

[G8HQP Dave]

Originally Posted by ms660

Which are the errors on the first page?

An efficient transmitting antenna must have exact dimensions.

Not really true. It may be a clumsy way of saying that a transmitter antenna needs to be carefully designed so that a good impedance match is achieved.

An ideal antenna has a definite length and a uniform diameter, and is completely isolated in space.

Not sure what they mean by "ideal". For theoretical calculations the ideal antenna is actually a long thin cone, not uniform diameter. For practical purposes the diameter may be thicker nearer the feedpoint (the opposite of the theoretical ideal) and the length adjusted accordingly.

A current flowing in a wire whose length is properly related to the rf produces an electro magnetic field.

An AC current flowing in a wire of any length produces an EM field. This seems to be pushing the popular myth that an antenna has to be of a particular size in order to radiate; the truth is that it has to be of an appropriate size so that we can efficiently couple to it.

Although a conductor is usually considered present when a moving electric or magnetic field is mentioned, the laws that govern these fields say nothing about a conductor.

No, EM theory covers conductors as well as fields. Where else would you have a current density as the source of a field apart from in a conductor?

It seems to have been written by someone with lots of experience in using antennas but a rather weak grasp of the underlying principles. Hence myth and truth are mixed together.

Originally Posted by crackle

G8HQP Dave, I dont understand your statement "Being short is not a problem." Maybe I am mis-reading that.
I know from practice and experience, at the MW & LW frequencies we are talking about here, that the longer an aerial (within reason) the stronger the signal pickup is.

I was questioning the statement that short antennas somehow pick up a greater proportion of noise, and therefore a MW receive antenna needs to be long - a significant fraction of a wavelength. In reality the antenna only needs to be long enough to pick up enough signal, and long enough to provide a sufficiently low impedance to match the receiver (at least roughly). Once you have achieved this then making it longer does not help.

Roughly speaking, the vertical height of the antenna is what picks up the signal (at least for local MW/LW stations) and the total length (including any horizontal top) sets the impedance so can help matching to the receiver. Note that when I say 'vertical height' I do not mean how high up the antenna is, but the vertical distance from the lowest point to the highest point. However, if the receiver is up in the air itself (e.g. in a bedroom) then the height of the receiver above ground can be added if it has a good earth connection.

[DoubleWound]

Originally Posted by Mike. Watterson

Lighting circuits are often the worst, with separate runs of live to the switch. Also above the foil backed plasterboard.

Yes, interesting points and I hadn't thought about foil backed plasterboard although it is not present in my house.

Today I isolated all circuits in the house except the cooker spur and I still got interference. Just don't know where it's coming from but it's not there in the back garden!

Playing around with my chimney mounted FM dipole today I found that the Marconi T29A reception and interference was improved to almost perfection over the range 200m-250m by connecting just the downlead screen to the set's earth socket (which is already mains earthed).

It's all weird.

[ms660]

Originally Posted by G8HQP Dave

Not really true. It may be a clumsy way of saying that a transmitter antenna needs to be carefully designed so that a good impedance match is achieved.

Not sure what they mean by "ideal". For theoretical calculations the ideal antenna is actually a long thin cone, not uniform diameter. For practical purposes the diameter may be thicker nearer the feedpoint (the opposite of the theoretical ideal) and the length adjusted accordingly.

An AC current flowing in a wire of any length produces an EM field. This seems to be pushing the popular myth that an antenna has to be of a particular size in order to radiate; the truth is that it has to be of an appropriate size so that we can efficiently couple to it.

No, EM theory covers conductors as well as fields. Where else would you have a current density as the source of a field apart from in a conductor?

It seems to have been written by someone with lots of experience in using antennas but a rather weak grasp of the underlying principles. Hence myth and truth are mixed together.

To be honest the first page made sense to me so I went to read some other pages, I've found it useful as a reference, here's another one I've had on file for some years:

http://www.esr.se/phocadownload/ante...0TM_11-666.pdf

Lawrence.

[G8HQP Dave]

These sort of pages play a useful role, provided that people realise that they have somewhat oversimplified things and are therefore willing to unlearn some of the misleading details as they progress in understanding. Otherwise they may become yet another propagator of antenna and feeder myths on websites.

[Mike. Watterson]

Conclusion, a ferrite rod or a loop (15cm to 40cm diameter) is simpler and less interference on LW/MW.
The ferrite is poor on HF and rubbish on VHF. A 1 to 3 turn loop will work on SW (depending on band), but at higher HF and VHF a whip is fine.

[ms660]

Originally Posted by G8HQP Dave

These sort of pages play a useful role, provided that people realise that they have somewhat oversimplified things and are therefore willing to unlearn some of the misleading details as they progress in understanding. Otherwise they may become yet another propagator of antenna and feeder myths on websites.

It's part of a navy training series NEETS and I thought it would fit in given the context of the thread and that maybe it would help some folks here on the forum.

I've had it on file on my PC for years and found it helpful and I know other folks have found it helpful too.

Here's a book link to the said, see pages numbered iii and iv near the beginning for an overview of the series:

https://books.google.co.uk/books?id=...0radio&f=false

Lawrence.

[Synchrodyne]

Originally Posted by Hybrid tellies

Gordon J King wrote an excellent book on TV and Radio aerials. Mostly aimed at TV, VHF and UHF, and FM aerials but I am sure there was a chapter on AM aerials. I will take a closer look at this over the weekend.

Yes, that’s a nice book, with straightforward presentation. AM aerials were covered. A couple of notable points quite neatly presented by GJK are (1) the basis for the earth connection (be it real or parasitic) for an open-wire AM aerial, and (2) the interference effects that can occur when a dipole is connected direct to a balanced feeder. Both aspects, but particularly the latter, have been neglected in some treatments.

Another useful book is “Receiving Aerial Systems” by I.A. Davidson. I think it was originally published by Heywood, and then republished in 1957 by Philosophical Library. It includes a worked example calculating the signal voltage available at the receiver from a 15 ft vertical looking into a range of impedances.

The attached article from WW 1949 March covered an effort by the Radio Component Manufacturers’ Federation to classify AM aerials according to their suitability for various field strength ranges. I suspect that these days, only Class A would have some possibility of being out of the worst of the interference field.




Cheers,

[colly0410]

A very interesting thread & I thank everyone for their input. I'm learning a lot reading it...

[Ian - G4JQT]

The BVWS Bulletin, Volume 26, Number 2 (Summer 2001) published a basic aerials article on page 26: "Installing aerials and earths - a forgotten skill?"

Unfortunately I don't have a scanner here or I'd post it on this thread.

[crackle]

Ian
It can be viewed on the BVWS website. Ill have a read of it later today.
https://www.bvws.org.uk/publications...letin_26_2.pdf
Mike

[Skywave]

Originally Posted by Mike. Watterson

The book (Radio and Television Engineers Reference Book, Hawker & Pannett) is very inaccurate.

Originally Posted by ms660

The book is very well respected.

Well it isn't by me.
Many, many years ago, when I knew a lot less about electronics generally, I regarded that book as a valuable source of information. However, as the decades passed and my knowledge & experience gradually improved, I found many technical errors in that book, some of which were mistakes of a very fundamental nature; many were simple mis-prints. The book was obviously not proof-read prior to publication.
I was amazed that Pat Hawker, G3VA, of all people had his name associated with that publication.
I no longer own that book, so cannot point to the many errors that I found in it.

Al.