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Ferrite Rod TX Antennas

Most of these tests were carried out at the old QTH some years ago.

No exotic claims are made for this antenna. I just tried a few experiments and was surprised with the results. What appears below may inspire you to try something equally "off the wall" to see what happens.


As a quick experiment today (Jan 31st 2013) I tried using my small ferrite rod as a TX antenna. The rod has about 80 turns of 0.5mm diameter wire wound on it for when it is used as a loading coil on 472kHz. At the "cold" end there are taps every few turns. With a 365pF air-spaced variable capacitor in parallel with around 60 turns and with the 50 ohm point one of the very low taps, I tuned it for 1:1 SWR on 40m using the FT817 at 5W. Tuning was VERY sharp, but manageable.

With a 2 minute WSPR-2 transmission I got an IMMEDIATE spot from a station in Denmark. This evening I tried again and got some spots from Norway with the best DX spot being from 2096km away!  The ERP must be very low but it appears to work as a tiny loop (area magnified as a result of the high ferrite permeability). At 5W from the FT817 there was no suggestion of core saturation and heating. The ferrite rod is operating on my table top with all other antennas totally disconnected.

Others have experimented with ferrite rods in the past and achieved moderate success. For example see G2BZQ's article in an old 73 magazine.

Based on "real" HF loop design, the best results with a ferrite rod should be with few turns of thick wire on the ferrite (less resistive loss), a high C-L ratio and a low loss tuning capacitor. Also, there is probably a maximum frequency that is likely to work well. 40m (7MHz) happened to be the band I tried, but I think the same results would still hold on 10MHz if the (unknown) ferrite properties are OK.

UPDATE: Feb 1st 2013

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 something too. What happens in the far field goodness only knows! WSPR can work with very weak signals too, so you don't need a lot of ERP to be copied. Mind you, some of the reports were strong -11dB S/N.

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.


UPDATE: Feb 2nd 2013

Today I managed to get the ferrite rod to match on 10MHz and got a decent number of spots around Europe. I have not managed to match it at 14MHz (yet) but think the ferrite rod material may be starting to degrade at this sort of frequency.


UPDATE Feb 26th 2020

Jay, W5OLF kindly sent me his 80m ferrite rod antenna. Although he had optimised it for 80m WSPR TX, it was way off tune and I had to add about 18pF to make it resonate. Neither Jay nor I understand why it was way off tune. Perhaps something caused the permeability to be altered on the journey?  On 80m WSPR RX it worked well although I only managed spots with 2.5W from 1 German station on TX.

As I have managed to find the ferrite rod and tuning capacitor used in my own tests some years ago, I may try my own ferrite rod on 40m or 30m WSPR TX again or FT8 TX.



Added Oct 27th 2018

Another person that has experimented with ferrite TX antennas is Giorgio Grisoni I2GSI from Italy. He has emailed me several examples of his antennas.  I have his email address and, if he agrees, I can share that with you. Of course, I will NOT share that publicly here. 

Some of Giorgio's articles are attached as files at the bottom of this page. These are in Italian, so you may have to use Google Translate to get the English versions!


The image on the left is Giorgio's 50MHz ferrite antenna.













Ċ
Roger Lapthorn,
27 Oct 2018, 11:28
Ċ
Roger Lapthorn,
27 Oct 2018, 11:27
ą
Roger Lapthorn,
27 Oct 2018, 11:27