Mag loop aerial for the Den

[Murphyv310]

I cant comment on whether you would need a grounded grid stage before each side of a valve long tail pair Jeffrey but it would be awkward to implement if the single turn loop were remote.  Using transistors its easy to arrange to supply the 12V down the coax cable but for vales ... well HT and then heaters ...

Gary

Hi Gary.
Totally agree that power supply will be a bit of a pain. Grounded grid would be the way to go but I think output could be low unless the stage was followed by another stage.
A cascode amp actually may be the best solution with valves and even with the transistors used here.

[ppppenguin]

I'm not suggesting anyone should build a valve pre-amp, just wondering if the technique would have been plausible before transistors. Cascode is a very good way to get high gain with low feedback capacitance.

The BD131 is specified as an audio device but like so many silicon transistors its fT is ample for MW.

[Eddystonepete]

[attachment=13985]Hi All, dont discount remotely tuning a loop. I built one using a model servo driven by a CD4001 giving 1 to 2ms pulses every 20ms. Worked perfectly and didnt cause any radio interference, it wouldnt of course given its original purpose. My intention was to put it on a tuned loop in the loft but then I managed to shift some stuff on Gumtree. Hello Wellbrook!

Peter

[Geordie McBoyne]

The tuned frame aerial I use consists of seven turns plus a single turn coupling winding all wound in a frame which is one meter square. Tuning is by four four BB510 varicap diodes in a series and parallel arrangement. The diodes are wired back to back. A single BB510 offers a wide capacitive swing with a applied voltage of 1 to 9 volts but I considered a single diode not to be a good idea because of the possibility of rectification of very strong signals. However, I am seriously considering replacing the diodes with a conventional tuning capacitor. Varicap diodes do offer the easy implication of remote control but I sure a vaned capacitor will be better.
The aerial is connected to my Pilot X65B. This USA made set offers excellent selectivity because of the triple tuned IF transformers. This feature was continued in many of the early UK made sets such as the U650.
Both these Pilot receivers have an RF amplifier.
One thing is sure there still plenty activity on the medium waveband, at least for the near future I hope.

Geordie McBoyne.

[pwdrive]

Here's a quick calculator for a single turn tuned loop:

http://www.66pacific.com/calculators/sma..._calc.aspx

Lawrence.

[Murphyv310]

Here's a quick calculator for a single turn tuned loop:

http://www.66pacific.com/calculators/sma..._calc.aspx

Lawrence.

Hi Lawrence.
The link is for a transmitting loop, at 1Mhz its enormous!

In fact doing the same calcs at 1Mhz its just about the same as our multi-turn frame aerial opened out to one turn!

[pwdrive]

The calculator will do for Tx & Rx, I wonder how the Wellbrook does the matching over a wide reactance range?

Lawrence.

[Murphyv310]

The calculator will do for Tx & Rx, I wonder how the Wellbrook does the matching over a wide reactance range?

Lawrence.

I doubt that a 1 metre loop is matched at all! At the massive size of the opened out frame aerial yes it will resonate at the desired frequency but the purpose of the Wellbrook and any 1 meter loop is to give good performance for the size and reduce mains borne interference. It has to be active though as the signal levels are very small indeed.

[pwdrive]

Wellbrook claims that the amplifier in their loop matches the loops reactance, hence the question in my previous post, I wonder if it's anything to do with this:

http://www.allaboutcircuits.com/technica...onverters/

Lawrence.

[Murphyv310]

Wellbrook claims that the amplifier in their loop matches the loops reactance, hence the question in my previous post, I wonder if it's anything to do with this:

http://www.allaboutcircuits.com/technica...onverters/

Lawrence.

Em!!!!!!!!!!
Not an easy one to answer especially as Wellbrook's amplifier circuitry is top secret. A few years ago I read an article that it was all very much the snake oil syndrome! I can't really comment on the amplifier but the simple two transistor amp works as well as if not better than my Wellbrook did when it was new.
Of course the acid test is to compare like for like, only then can we make judgement.
I am going to build a 2 metre single turn loop and possibly a 2 metre circular multiturn loop for the crystal set projects.
I'm also planning a meet for the 29th of May ans it would be good to get others input on all of this and do some tests too.
Right now it's all down to health though as to the construction projects and the meet up.