Building an external ferrite rod aerial

[paulsherwin]

It's been at the back of my mind for some time to build an external ferrite rod aerial, primarily for LW use, so that radios designed for an external aerial can be used without one. This should be very easy to do - maybe just a coil and variable capacitor, or perhaps using a transistor to boost the signal a bit. I can't believe nobody has built one of these before me, but can't find any reference to such a thing.

Possibly I've missed something important, and it's actually very difficult to make such a device work. Does anybody have any info or suggestions before I just blunder in?

TIA, Paul

[paulsherwin]

Thanks for the googling advice Jim, that did indeed find more useful hits than my previous searches. I never knew that Americans call ferrite rod aerials 'loopsticks', but I guess this is common knowledge for any radio amateur.

I didn't find precisely what I was looking for, though. I'll have another look tomorrow after a night's sleep!

The idea is to build something in a small plastic / wood box which can sit on top of or behind an old radio unobtrusively and be plugged into the aerial and earth sockets. A non-ferrite tuned loop would be a bit big.

Best regards, Paul

[JHGibson]

Paul,
There was a construction article in April/May 2001 Radio Bygones that sounds perfect for your requirements. Basically just a two gang tuning cap tuning a hand wound coil on a 8 inch ferrite rod. 8 inch ferrite rods are rather rare but you could glue two 4 inch rods together, it will work just like a 8 inch one if the glue joint is thin. The tuning covered long and medium wave.
John.

[paulsherwin]

Thanks for that John. I don't get Radio Bygones and don't have access to the article, but if somebody could scan it and email (even just the circuit) I'd be grateful.

In the meantime, in the absence of precise guidance I'm going to cannibalize a scrap trannie, hook up a high gain transistor to the low impedence winding and see what happens. Watch this space. If I ever get it finished properly I'll write it up and Paul can put it in his 'projects' section.

Best regards, Paul

[quantum]

When these type of boosters used to appear in the 'Take Twenty' column in early '70s PW, Julian Richards made use of either a BC108 or a 2N2926 (seemed to be his favourite transistors) - how about using an AF117 for added oldy-worldy authenticity?

[paulsherwin]

Originally Posted by quantum

When these type of boosters used to appear in the 'Take Twenty' column in early '70s PW, Julian Richards made use of either a BC108 or a 2N2926 (seemed to be his favourite transistors) - how about using an AF117 for added oldy-worldy authenticity?

Ahh, Take Twenty, that takes me back. I think those transistors were used because they were the cheapest silicon transistors you could buy at the time that would actually work. I've got some ripped out Take Twenty pages in a file somewhere, I must dig them out. I don't remember a specific design for an amplified external ferrite aerial though - maybe I missed that issue, or was too busy taking O levels etc.

Best regards, Paul

[Robert-UK]

Paul -

I can confirm there was nothing about external loop aerials published in the PW "Take 20" column.

However, in addition to the Radio Bygones article (which is indeed what you want) you might check the following:

Everyday Practical Electronics, September 2000
Practical Wireless November 1966 (MW loop)
Practical Wireless April 1973 (MW loop plus transistor amplifier)
Practical Wireless November 1979 (MW/LW loop aerials)

However, all the PW offerings are frame aerials, not ferrite rods.

Regards

Robert

[paulsherwin]

Well, I just lashed a prototype together, and it worked. I used the following components:

Ferrite rod, coils, variable cap and w/c switch from Decca trannie circa 1962

2SC1364 transistor ex TV IF board

1M and 4.7K resistors, 2 * 0.04uF, 1 * 47uF caps ex junk box

Coax socket ex junk box, PP3 battery

The result pulls about 1ma (as you'd expect) and reception of RTE 252kHz is excellent using a 1949 Ferguson. It really is that easy, I don't know why I didn't build one before. Now I just have to build it properly in a nice pretty box.

Thanks for all responses to the thread, Paul

[Alan_Douglas]

This one covers 9-13, and 12-40kHz. The only shield is the aluminum box cover. The coils are litz but I doubt that it makes much difference at these frequencies:

http://antiqueradios.com/albums/Misc/FERRLOOP.jpg

[Ed_Dinning]

Hi Gents, were the coils wave wound? if not such lage coils may have a lot of self capacitance that would reduce the Q of the tuned circuit.
Ed

[Hybrid tellies]

What a brilliant original idea and as Paul has proved the building of one should not be a problem for any of us as we should all have suitable parts lying around our houses.
The other advantage is, thanks to the directional properties of the ferrit rod aerial, some local sources of interference and noise may be reduced or even eliminated just as you can do with a portable set.

[paulsherwin]

I've now built a permanent version in a wooden box, using pretty much the same components as the prototype. I hand selected a higher gain transistor (hfe 600, prototype was 300) but this doesn't make a lot of difference and any general purpose small signal silicon transistor should work (such as a BC108 or 2N2926 ).

I've just spent half an hour listening to BBC Radio Cambridge and some Dutch music station. It isn't really suitable for DXing though, since the tuning is quite precise and if it isn't tuned correctly you can't hear the DX station! Pulls in RTE R1 like a good 'un though.

I can recommend this easy and zero cost project to anybody without a good longwire aerial. Here's the circuit, such as it is (wavechange and on/off switch omitted).

Best regards, Paul

[Ed_Dinning]

Coil details please Paul, or have I missed them somewhere?
Ed

[paulsherwin]

Sorry Ed, this thread has rambled a bit. The coils are standard MW and LW coils from an ancient scrap trannie. These particular coils have a tap about 90% along the winding, but you could use coils with a separate low impedence winding easily enough. Only one coil is shown in the circuit diagram - the wavechange switch switches the connections to the transistor base and the variable cap.

The only drawback with using scrap trannie coils is that it's sometimes difficult to work out which connection is which. I got the MW connections wrong at the first attempt.

There are probably better designs around than this - I basically made it up as I went along, and have stuck with it because it works.

Best regards, Paul

[BlowFish]

Is J1 the connection to the radio ?
C1 standard tuning cap ?

[paulsherwin]

Yes, I knocked up the circuit diagram quickly in CircuitMaker and it's a bit terse. C1 is a variable cap (I used a small one from yet another scrap trannie). J1 is a coax socket, though you don't have to use this - I guess you could use flying leads or whatever.

It really works!

Best regards, Paul

[dave_n_t]

Hi Paul (Sherwin),

Do you have any trouble with instability?

The only way I can get the thing to stop oscillating (e.g. as evidenced by fairly broadband whistling on the SW bands) is by specially selecting a trannie for low gain

Other things I've tried:

0.1uF power rail decoupler close up to Tr1
100ohm blocker in series with output
reduce power supply to 3v
change bias resistance to 100k (more negative feedback)
earth tap or bottom of coil (feeding base thru' 0.02uF)
feeding from hot end of tuning coil (thru' 0.02) [don't worry about loading the coil - slightly damped response wd probably be easier anyway!]
output to coax vice hook-up wire

I'm running out of ideas!

dave

[paulsherwin]

Originally Posted by dave_n_t

Hi Paul (Sherwin),

Do you have any trouble with instability?

My mental health's none of your business

The transistor does provide a lot of gain. You can get oscillation if you introduce positive feedback, such as by adding capacitance between the transistor collector and the top of the coil or by using a long length of unscreened wire for the transistor output.

I did get some instability when I tried to increase the gain by using a high gain transistor and increasing the bias resistor to 2.2M, but using 1M cured this.T here's nothing especially convoluted about this - the electronics are mostly wired to the lugs of the on/off and w/c switches.

Does the circuit still oscillate if you disconnect the output cap from the transistor collector?

Best regards, Paul

[paulsherwin]

OK, I've investigated a bit further. The unit tends to oscillate if it's connected to a radio with a floating earth, especially on MW. This is presumably because the coax shielding is able to radiate RF if it's not grounded.

However, I've found that it oscillates at the HF end of the MW band, even if the radio *is* earthed. The middle and the LF end of the band are fine, as is LW.

I'll try to work out what's going on and report back.

Best regards, Paul

[paulsherwin]

I've tried running the unit on 3V and all signs of instability disappear. This is probably just a question of reducing the gain.

3V may be a better idea anyway. With 9V, the gain is so high that AGC circuits struggle with the signal strength on local stations like R4 or R5. I suspect that this much gain requires proper RF construction techniques to prevent instability, rather than just lashing a load of junk box parts together in a wooden box (which is what I did).

Of course, I could use 9V if I used an OC45 transistor... Hmmm...

Best regards, Paul