1 Feb 2013

More Ferrite Rod TX experiments

Vertical orientation today - good results on 40m WSPR TX
This morning I tried to get a few more WSPR reports on 7MHz using my small ferrite rod antenna. I managed a few only, then after about 20 minutes I decided to try again with the ferrite VERTICAL on the desk. Don't ask me why I did this, but then I started getting a large number of spots!
Now, all other antennas were disconnected and I tried this arrangement in several places around the room and ALL got decent spots. Maybe what I have here is a bit of a hybrid. If you look at the diagram there are about 80turns on the rod beyond the parallel tuned circuit that I assumed was the magnetic loop doing the radiating. Then again you can consider the parallel tuned circuit as a base loading for the 80t short vertical above it. Tuning C1 will bring the whole system to resonance and a low SWR can be found by adjusting the tap point, which turned out to be best very close to the bottom, about 1 turn up. There is no earth connection. I make no claims as to how this works. Some suggest it is just an elaborate coupler into the house wiring, but the very sharp tuning makes me think this is not likely. Also, if this was the case then surely moving it around in the room would make a big difference? Below are the spots for a few hours on 40m.
WSPR spots with the antenna above on 40m today
However it works, it manages to do pretty well. Now I do not believe in "snake oil" antennas, and make no claims for this one: a small ferrite rod will radiate something (H-field). Add a ferrite loaded vertical as well (an E-field antenna) and that will radiate too. What happens in the far field goodness only knows!

When the weather is better I will take the whole kit into the back garden well away from the house and repeat. If it is working without coupling into the house wires (as I think is the case) then results should be comparable. If spots disappear then it will have turned out to be a very good random wire coupler, HI.


Peter Marks said...


this is really fascinating. I'd love to see comparisons between the ferrite rod, a small loop and perhaps a quarter wave of wire just strung around the room.

WSPR is a great way to compare antennas and you're really on to something here.


Anonymous said...

Its bound to tune sharp if its a tapped down L/C wid no load on it!

Roger G3XBM said...

That could be the case of course Tony, but ANY magnetic loop TX antenna, if losses are low, has sharp tuning. A high unloaded Q is essential to keep the losses low. Considering the Rrad as a very large parallel resistance across the loop, the Q will be halved when optimally matched, so the Q will still be large. I am NOT saying this antenna is very efficient, just that it radiates a signal rather better than (I) might have expected.

Bob G3WKW said...

I also spotted you but went unreported as I was messing about with WSPR too much. I notice no spots below 150k. Would you have had closer spots on a "nornal" antenna. Also 50 milliwatts I guess was your RF input as you made no assumptions about the gain of the antenna. It would be interesting to increase power and see the increase in reception. The ferrite must have a saturation level.

Roger G3XBM said...

Bob, the 50mW is a notional level of RF ERP. I am using 2.5W from the FT817 (3 blobs) into the ferrite rod with no signs of saturation. The efficiency of such a small loop (if that is indeed what it is) is going to be very low indeed. I really must find a way of measuring the actual ERP. Maybe I could get a local to do a comparison between my 6m vertical tuned to 40m against the ferrite antenna. I could get the S/N difference and work back to ERP from the loop.

Sverre Holm said...
This comment has been removed by the author.
Sverre Holm said...

There is a paper in 73 magazine, March 1999 with title "Transmitting ferrite loop for 80/160" by G2BZQ. It can be found in the 73 magazine index. There is some parallel experience there.


Bert, PA1B said...

Hello Roger, At first I was puzzled by the fact, that your Blog mentions 5 Watts and I see 50 mW in the WSPR datatbase in the first WSPR spot.
I think that the efficiency can be more than 1/100. A quick calculation reveals that the ferrite rod of 15 cm can be compared with a lop of 30 cm in diameter. 73, Bert

Roger G3XBM said...

Bert, remember I keep forgetting that "3 blogs" on the FT817 is 2.5W, not 5W! So for all these ferrite rod tests the power from the rig was 2.5W The 50mW (ERP) figure is my guess (-17dB sounds a probable gain). I need to find a way of measuring it. Measure the antenna current and working out the effective "in air" equivalent of the ferrite's diameter. Square root of mu times bigger? So if the effective permeability was 50 the loop would be sqr root of 50 x pi x 15mm? My maths may be wrong: it was a long time ago I worked this sort of thing out, HI.

Anonymous said...

I was always taught that the coil would produce just a local inverse cube law signal - associated with just a magnetic field.
BUT THEN a moving(oscillating)magnetic field in air will induce an electric field at right angles - 180 out of phase.
Thats a Radio Wave!
So you might not have just coupling to wires

Roger G3XBM said...

Tom, I am pretty sure this ferrite structure radiates in its own right. Loop antennas work, and the ferrite rod, with all its high permeability focussing, is just a loop when all is said and done. In my case it is a magnetic loop plus a small loaded E-field vertical.

Anonymous said...

Roger - Yes i tend to agree..