Wideband regenerative (ferrite rod) loop antenna

Hello,
I have been thinking about this project for a long time. The more I think about so the more I think that I really want some form of efficient transmiting antenna, and that cannot be used regeneratively.

If you look at the 20m loop antenna on my homepages, I find that the antenna itself has a bandwidth of something like 100kHz. The 80m version had a bandwidth of about 20kHz. The 80m version noticeably added a slope to the audio if it was even slightly off-tune.

But I can see a great use for having one antenna for transmitting and another for receiving, if the TX-to-RX levels can be brought down to a safe level. I can also see this having great uses for suit-case receiving. I have done a lot of travelling in my days. Just to listen to the BBC World Service was often a luxury. Certainly worthy of some experimentation.

I wonder if one could make a SW AM or SSB/CW receiver based on a regenerative antenna?

Many years ago (1969) I had a roll of thin insulated wire on a bobbin, with a 1/2" hole down the center. I guess there were a couple of 1000 turns of wire on the roll. I pushed a ferrite rod through the center, then extended both sides using two halves of the ferrite core from a TV Line Output Txfmr. The whole ferrite rod was then about 30cm long. I used it to receive the audio spectrum in the barrack block bathroom. The sender was my record player with a double 2.5m Dia. 11-turn wire loops (2 for stereo). I got a range of about 25 metres. But this was broadband.

Best regards from Harry - SM0VPO

Here's an interesting article (by NASA, no less) about regenerative loop antennas, including a wideband variant. The key is to decrease the system's total impedance, in order to increase loop current, in order to allow the loop to interact with a larger area of the physical EM field.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940020710.pdf (Note: the author of this document is a Mr. Sutton, who also was referenced in the first post of this discussion thread.)

Regeneration makes an antenna larger in a physical sense because, with added regeneration, the antenna is interacting with a physically larger area of the EM field.

I was able to regenerate a 20 cm x 20 cm loop of hookup wire to get good indoor reception of strong shortwave broadcasters on a small portable receiver placed inside the loop. Regeneration can turn such a hopeless antenna into a usable one; it can also turn a high-Q loop antenna into an even higher-Q antenna with correspondingly increased effective height (effective area). Of course, there could be problems with overload, IMD, etc., and also it is tedious to adjust the regeneration constantly when you change frequency -- but regeneration does seem to be a kind of "magic bullet" that is useful for extremely limited antenna situations such as indoors or in very small spaces.

And yes, I've regenerated ferrite rod antennas also, with reasonable results (reception of some strong shortwave signals).

Here's another experimenter who had great results with a regenerated ferrite rod: https://www.hfunderground.com/board/index.php?topic=41909.0


Is anyone else experimenting with regenerative loop antennas?

---

After re-reading this thread, I think the original poster was specifically asking about the wideband, untuned "regenerative" antenna by Mr. Sutton. The famous Russian ham Polyakov (RA3AAE) also has commented on this scheme. See http://theradioboard.com/rb/download/Polyakov-Regen.pdf (translated from original Russian as referenced at http://theradioboard.com/rb/viewtopic.php?p=53386#p53386). I don't know if Sutton's broadband antenna scheme will work at HF or not (he specifically described an ELF system). It is an interesting question. I do know that normal regenerative antennas work great at HF.

Note that Polyakov also proposes a broadband regenerative antenna in Figure 4 of his above document. In contrast to Sutton's circuit (which worked at ELF, and used op-amps), Polyakov's circuit uses only 2 discrete transistors, and is claimed to work at HF. The Russian article is here: https://litresp.com/chitat/ru/Ж/zhurnal-yunij-tehnik/yunij-tehnik-2008--08/18 . However, after reading the article a few times, it seems Polyakov's circuit is supposed to work with a whip antenna, and seems intended to cancel the detrimental effects of connecting a coaxial cable to a short whip antenna. It may not be related to regenerative loop antennas.

Hello,
I think that the links to the pictures are not current.

I remember in the 1960's I had a Philips Electronic Engineering set. One of the projects was a Medium Wave reflex receiver. That used a ferrite rod antenna. Lovely little AM receiver. The transistor (AF114) amplifier the RF, diode detector, and the audio fed back into the same transistors - "Reflex Radio".

I put a twisted-wire capacitor from the collector of the AF114 and connected it to the "hot" end of the tuned winding on the loopstick. That increased the Q-factor so that it performed more like a superhet. I also took an RF output from the collector and fed that into my SW radio. It made a lovely active antenna. With 28 turns  +  2 turns on the loopstick it covered the "trawler band".

I had that for years to listen to 160 metres. The receiver by itself was ok for broadcast stations, but for HF it was not sensitive enough. Using it as a portable antenna was really great. The ability to tune the antenna also cut out a lot of overload from nearby stations.

I have often thought about that radio since it was so simple and gave really surprising results.

I never thought about remote tuning, in fact, in those days I had never even heard of varactor tuning.

BR Harry

I'm working on a multiband regenerative loop antenna with electronic bandswitching. You can see the current design here: http://theradioboard.com/rb/viewtopic.php?p=93321#p93321 .



I have currently built a simpler version of that design, without electronic bandswitching, as shown here: http://theradioboard.com/rb/viewtopic.php?p=93267#p93267


My design incorporates varactor tuning and remote control (via long control wires) of tuning and regeneration. It should be possible to control tuning and regeneration via computer, e.g. with a Raspberry Pi. Furthermore, once the electronic bandswitching is working, it should also be possible to control bandswitching via computer. This would make a "universal" regenerative loop antenna that can be fully controlled remotely.

I would be interested in hearing the results if anyone builds this or a similar design. Regenerated loop antennas really work surprisingly well for their size.

Hi Glenn,
That sounds like a very worthwhile project.
At the moment I am rather busy in Spain (no radio this time) and when I return to Sweden time will be short.
I am presently building an oscilloscope and boxing the 0-30MHz DDS signal generator.

After that it could be an idea to make something similar to your suggestion.
In the 60's I did a lot with Q-Multipliers (first with 6C4, then with AF114). Worked a treat, but a little positive feedback can increase sensitivity, and if the components are chosen correctly possibly a TX/RX antenna, with regeneration.
Let me think about that for a while.

Anybody else intrerested in taking up this challenge?

BR Harry

Hi Harry

I have a "challenge"...

It is possible to built a regenerative wideband ferrite antenna - maybe up to 30MHz? (with e.g. magnetic material 61; page 17-18 ( http://web.archive.org/web/20170806130801/http://www.g8jnj.net/Balun%20construction.pdf ) or (2)). Please note that the ferrite rod can be removed - and then you have a wideband regenerative loop antenna.

There are schematics and good description of a regenerative wideband ferrite antenna in the following patent (1). E.g. use op-amp LMH6647MF (-3dB at 55MHz).

Do you or others want to try to built it?

-

(1) Patent:

Patent seems to be close to be expired; 23 years: 1994: Wideband ferrite full shortwave-antenna:
The positive feedback provided by the feedback current lowers the antenna impedance which, in turn, increases the effective area of the antenna:
https://patentimages.storage.googleapis.com/pdfs/US5296866.pdf
Quote: "...
The outstanding stability of this active antenna
...
The reason why an antenna with regeneration has greater sensitivity than one without regeneration may be understood in terms of the concept of antenna “effective area.”
...
It is yet another object of this invention to provide an improved broadband active antenna system.
..."

(2) Possible ferrite rod:
https://web.archive.org/web/20170212031426/http://www.schubert-gehaeuse.de/prod01.htm
Quote: "Ferritstab bis 30Mhz verwendbar".