This morning I tried the ferrite rod antenna on 10MHz WSPR TX for an hour. As before, I mounted the ferrite rod VERTICALLY on the wooden desk with all other antennas disconnected - the only other antennas are a VHF colinear and a 10m halo, and both of these are around 10m away from the shack. Quite a respectable number of spots we obtained with 2.5W from the FT817.
So far I have not managed to get the ferrite rod antenna to match on 14MHz. It may be the losses in the ferrite are starting to impact performance?
I am still unclear how this tiny TX antenna is working: the assumption is a combination of a small amount of radiation off the H-field (ferrite rod coil) loop and some E-field radiation off the coiled vertical section above the tuned part. Then there is still the possibility that it is just acting as an efficient coupler into the house wiring that is doing the real radiation. Whatever, it puts a signal around Europe and occupies a volume with the tuning cap of just 15cm x 10cm x 5cm on the wooden shack table.
9 comments:
I was wondering how this would work on the higher bands. I have to try this for myself. I've already sent off for a couple of ferrite rods on Ebay!
Julian, G4ILO
Julian, I make no great claims for this but it does seem to work. My own ferrite is of unknown type but is 15cm long x 15mm diameter. It is rather fun to operate with such a tiny tiny antenna on the desktop. I should put it in a plastic box with a resistive matchbox and current meter really, although this wasn't meant to be a real project: I just tried it for fun and then got on a roll with it, HI.
New application for ferrite rods. TX antenna.
73 Ron PA2RF
Hi Roger,
Interesting - I suspected some other radiation mechanism was occurring other than the 'magnetic' component.
I have seen designs for ferrite loaded normal mode helical antennas. But they tended to have a lowish feed impedance.
Why not build a conventional high Z matching network in a screened box. The place the ferrite helix on top of the box. This would isolate radiation from the matching network, and give an indication WRT how much radiation was due to the magnetic vs. electric components.
Can you determine what ferrite mix your rod is using ? It is likely to be either 43 or 61 mix. 61 mix is more suited as it will introduce much less shunt resistance. However it wil need a lot more turns to operate on the LF bands.
The main problem with ferrite loaded antennas is the power handling. As they produce an open magnetic circuit (otherwise it wouldn't radiate). So an average size rod will saturate fairly quickly. Heat up and loose permeability. I wouldn't expect to be able to run more than 5 to 10 watts max.
Looks like you have created a ferrite loaded E-H antenna!
Sceptical Moi ?
Regards,
Martin - G8JNJ
Found the patent I was thinking of:-
http://www.google.com/patents/US4270128?printsec=drawing#v=onepage&q&f=false
Regards,
Martin - G8JNJ
Here's a practical compact HF antenna
http://www.google.com/patents/US7737905?printsec=abstract#v=onepage&q&f=false
However it needs rather a big ferrite rod 96" x 24" !
Some other work comparing compact antennas for underground communications on the LF bands can be found here.
http://stacks.cdc.gov/objectView!getDataStreamContent.action?pid=cdc:8885&dsid=DS1&mimeType=application/pdf
Regards,
Martin - G8JNJ
Hello Roger,
may I suggest you to keep your antenna on 10 MHz. Go to an open field with the ferrite attached on a support and try WSPR.
If we get something on the WSPR website then your ferrite is really radiating... if not, it's coupling in your house wiring.
73 from Thierry.
Hi Roger,
After doing some more reading I found this comment in a BBC white paper.
"However, because the antenna is tuned, its circuitry possesses a very high impedance and is therefore susceptible to E-field pick up; that is, the loop doubles as a short whip antenna. "
http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP091.pdf
It's obvious when you read it - but I hadn't thought about it in that way.
A lot of the serious MF /LF beacon DXers seem to put a Faraday screen around their ferrite rod antennas to prevent this happening. Either copper foil with a gap so there isn't a shorted turn winding. Or ribbon cable arranged as interleaved grounded 'fingers' wrapped around the rod and coupling coil.
Regards,
Martin - G8JNJ
Interesting! In my limited experience with small mag loops and ferrite bar loops, they're most efficient on 10 to 20 MHz and usually mediocre to lousy on 7 MHz. Wonder what yours would do on other bands. NH7L
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