Loop Aerial

[Murphyv310]

Thanks for that link.

I don't understand how it is a magnetic aerial although he uses the term.  To me it has to be an aerial shielded from the E part of the received signal.  So shouldn't it be a wire inside a tubes as per the Wellbrook?  Wonder how they get the ferrite inside the tube and what difference that makes.

In the Den I have a large window and I could put a tube all the way around it with an inner wire, coupled to the amplifier. Any ideas if that would work?  I understand that shape is not critical, it doesn't have to be round and larger presumably means more signal?  With the Wellbrook, for the lounge best radio, it is on a rotator bought NOS for a very low price but for MW and LW I don't find it any advantage so having a window aerial fixed in a single plain wouldn't matter.  Actually the Wellbrook started off  on the window sill of the Den and worked very well for the radios in there, with large signals.  This was also the case when tried with the HMV 650 just loose in the lounge.  LW R4 closed the eye tube with considerable overlap but now garden mounted and 60 feet away signal is down and eye not fully closed.  That just must be the loss in the RG59 coax.

thanks for ideas   Gary

Hi Gary.
I'm sure you will find all the info you need on how magnetic aerials work on the internet.
It would take up a lot of keyboard time for me to explain it.
Have a look here: http://sidstation.loudet.org/antenna-theory-en.xhtml
My main gripe with the wellbrook was its signal output, a higher gain amp would have been useful, later models did have a bit more but even then it wasn't great.
When I "repaired" mine a few years ago with the circuit depicted in the thread signal levels increased although the loop didn't work up to Band 1 after.
I actually received some excellent Band one TV DX early on but there is little point now as we are bombarded with "Digital is best" and there is virtually no analogue TV DX to be received now.
The latest incarnation though does perform better and is working well up to the 10m band.

Long coax cable runs shouldn't give any noticeable losses on MW and short waves, most likely the problem is impedance matching, a 50 ohm cable is hardly likely to match an old valve radio aerial connections that are high impedance.
Try making a little matching transformer with a ferrite ring at the radio's A & E sockets, I've found this helps no end.
Good luck

[Radio Fixer]

"Long coax cable runs shouldn't give any noticeable losses on MW and short waves, most likely the problem is impedance matching, a 50 ohm cable is hardly likely to match an old valve radio aerial connections that are high impedance.
Try making a little matching transformer with a ferrite ring at the radio's A & E sockets, I've found this helps no end."

Its odd though Trevor as there was much more gain with the aerial on a short coax to the radio, so unless I damaged something with say a short on the line, when fitting the BNC's and having a little trouble, then it must be cable loss. I did try a balun and it had no effect, making reception worse ... what is the impedance of a broadcast radio anyway and it isn't constant across the bands.

Will have a read of the link.

My Wellbrook is good, being a later high gain version, and I can listed to R4 for example day and night completely interference free.

thanks Gary

[Murphyv310]

My Wellbrook is good, being a later high gain version, and I can listed to R4 for example day and night completely interference free.

thanks Gary

Hi Gary.
The only difference that should be noticed is the signal level between the old and re-designed Aerial. The signal to noise ratio will be virtually the same.
Where the old design fell down was when you were trying to resolve a weak station on a rather insensitive radio. A comms set or a decent set with say an RF stage or even an extra IF amp, there was never a problem, in fact the early loop has one distinct advantage of not getting overloaded with very strong local signals. I do know of one user that has this problem.
I really think the modification is the best compromise, it has better output to the original set up and still is good with strong signals. For all we know what's in the sealed box of Mr Wellbrook's loop could be similar if not the same as the the two transistor amp here.

Regarding your signal loss over a long length of coax, the effects of 50 or 75 ohm coax will be more noticeable at the terminating end of it, I think you do need to make a simple RF step up transformer.

[Terry]

Regarding your signal loss over a long length of coax, the effects of 50 or 75 ohm coax will be more noticeable at the terminating end of it, I think you do need to make a simple RF step up transformer.

I will second that.

Gary, for the answer to your question as to why so much signal is lost in the unmatched cable, look up a data sheet for RG59 (or whatever cable you are using) and find the capacitance per metre.

Now calculate how much capacity there is across your unmatched feeder!

I am reminded of a club who wanted a second extension microphone cable for their PA. Someone found a reel of unwanted coax and used that.

Very little above about 200Hz came out of the other end! It was, of course, a high impedance microphone ...                           

[pwdrive]

For your average valve radio try a 1:9 transformer, a ferrite toriod job, plenty on the web about them.

Lawrence.

[Murphyv310]

For your average valve radio try a 1:9 transformer, a ferrite toriod job, plenty on the web about them.

Lawrence.

I'd go with that.

[Radio Fixer]

Tried it with a transformer from China and it made no difference ... think it was worse but I will give it another go.

   

How can it match when the aerial input is so indeterminate for a domestic set?

But thanks Gary

[Murphyv310]

Tried it with a transformer from China and it made no difference ... think it was worse but I will give it another go.



How can it match when the aerial input is so indeterminate for a domestic set?

But thanks Gary

The item you have is to match a long wire which is high impedance to a radio that's low impedance. Basically a reverse of what you are trying to achieve. Try connecting it in reverse, hopefully it'll work.

[pwdrive]

Gary, the one in the link in your previous post looks like it's a long wire antenna balun, not sure how it would work in reverse, a proper 1:9 transformer (50 ohms in 450 ohms out) should match reasonably well to a valve receiver with antenna and earth inputs, most sig gen matching pads for radio RF alignment were around 400 ohms plus a bit of capacitance and were designed to simulate the average short wire antenna used with most radios back then so far as I remember, some of the old comms receivers had balanced antenna input terminals designed for 400/450 ohm ladder line

Lawrence. EDIT: Whoops.......post crossed.

[Murphyv310]

Hi.

TBH the impedance tends to vary with valve radios from 200 to as much a 1000 ohms.
Any step up RF transformer from 6:1 to 12:1 will work, often trial and error will give the best result.
Mind this is an RF matching transformer not a Balun! Here's one I lashed up this afternoon, its about 6-7:1 works a treat!