70% smaller antenna for cave radio?

D

Dave H

Guest
I thought that this might interest any CREG members or radio hams out there.

A research engineer at the University of Rhode Island has invented an
antenna 70 percent smaller than conventional designs, but which has
comparable sensitivity and increased bandwidth. The antenna, called a
distributed-load monopole (DLM), uses a helix and a load coil to shrink the
size of a normal quarter-wave monopole. In testing research engineer Rob
Vincent's antenna design, which cancels out the normal inductive loading,
the U.S. Navy found that the antenna achieved equivalent performance with
antennas 30 to 70 percent shorter than an ideal quarter-wave design. Read
more:
http://www.wirelessnetdesignline.com/showArticle.jhtml?articleID=163103803

Now I've looked at the U.S. Navy's report and it says that they didn't test any antenna designed for less than 7MHz. But that the ones they did test worked comparibly with normal 1/4 wave designs.
If you can remember a few years ago, there was an aftermarket car aerial kit that had a helix at the bottom before reverting to a more conventional stick design. I understand that these new designs are similar except that the bottom of the stick part has been folded into the helix part.

The CREG group have been using ribbon cable over a wooden frame held vertically in a number of their experiments (it's especially good for locating null points above an underground transmitter, for instance). If one of these was extended (like pulling open a spring) and connected to a normal dipole aerial running through it's centre, then this may approximate to the test antenna.

I'm sure if R alph J obson or D ave G ibson are on this forum then they will tell me of the flaw in the idea - but I can't think that it wouldn't be worth trying it one day.
 

SamT

Moderator
:crazy:
interesting stuff.
We relied on cave radios (hayphones) in the Berger last year and were let down (mind you - the only party that were). Pretty bloody complicated business. but very handy if they work.

as an aside - I know Ralph is not on the forum (his broadcast messages for the Peak are nomally posted by someone on his behalf) As for Dave - Im not sure.
 

paul

Moderator
SamT said:
:crazy:
interesting stuff.
We relied on cave radios (hayphones) in the Berger last year and were let down (mind you - the only party that were). Pretty bloody complicated business. but very handy if they work.

What probelms were you having with the Heyphones? Where were they located, out of interest.

When we were there 2 or 3 years ago Paul McCrilly (i think that's how it sis spelled) was fooling around with a System Nicola set up and that seemed to work really well. He had somebody with a transmitter/reciever on the surface near teh entrance and you could hear him clearly as he progressed through the cave, at least as far as the bottom of Aldo's.

SamT said:
as an aside - I know Ralph is not on the forum (his broadcast messages for the Peak are nomally posted by someone on his behalf) As for Dave - Im not sure.

I think he means Ralph Jobson, not Johnson!
 
Dave H said:
I thought that this might interest any CREG members or radio hams out there.

(Dave H's quote snipped)

The CREG group have been using ribbon cable over a wooden frame held vertically in a number of their experiments (it's especially good for locating null points above an underground transmitter, for instance). If one of these was extended (like pulling open a spring) and connected to a normal dipole aerial running through it's centre, then this may approximate to the test antenna.

I'm sure if R alph J obson or D ave G ibson are on this forum then they will tell me of the flaw in the idea - but I can't think that it wouldn't be worth trying it one day.

I do not usually (in fact, hardly ever) read this forum, but I just popped my head around the cyber-door to see what was going on.

To answer your question.... Unfortunately, it would not be worth trying.

The antenna in the report is a completely different type of thing to a cave radio antenna. The report says "quarter-wave dipole", so it is a resonant electric field antenna. The problem with these antennas is how do you persuade the electric current to travel all the way to the end before 'leaking' out into space. Left to its own devices, the current will leak out well before the end, so the antenna behaves as if it is shorter than it actually is. The report you quote describes a method of ensuring that the current does go all the way to the end. There are other methods - radio amateurs will talk about "top hats", on the end of the wire, for example.

The antenna is a quarter-wave dipole - that is, it would be 40mm long at 1800MHz (GSM frequency) or 750mm at Band 2 broadcast FM frequencies, or 2.8m at 27MHz CB radio frequency. But at 100kHz cave radio frequency, a quarter-wave dipole would be 750 metres!

Induction cave radios, with ribbon-cable antennas, as you describe, are not electric field antennas (wires with only one end connected to the PA) but magnetic field antennas (loops of wire with both ends connected). Moreover, they are not resonant, because they are far too small. (A self-resonant loop would need to be 950m in diameter at 100kHz). So, basically, a whole load of different principles come into play, and the report you mentioned is, sadly not applicable.

You mentioned ribbon cable on a frame. That's convenient to build, but it is not necessarily the best design. In fact, one of my "complaints" to CREG members is that nobody has ever actually designed a loop antenna - they just throw a few turns of wire together and expect it to work. Well, it does, after a fashion, but I assert that there are quite significant improvements that can be made to loop antennas, if only designers actually thought about it properly. (OK - that's a bit disingenuous of me, I guess). In a sense, we've been side-tracked by the HeyPhone, which uses an earth-current system; and which gave such a significant improvement over the Molefone (with its loop antenna) that people did not stop to consider that it might just be a case of redesigning the loop antenna. That being said, an earth current system is probably always likely to out-perform a loop system to some degree. I just question the degree.

So, although that report is not applicable to cave radios, it is true that <I>some</I> redesign of a loop antenna would be productive.
 
D

Dave H

Guest
Thanks Dave.

I'm sure I'll get back into the CREG at some point, but you can probably guess that my antenna design knowledge is based on an hour lecture 20 years ago (which I have forgotten). Even back then my telecommunications course was more about wired and optical systems rather than radio, so I suppose it's a bit of a dying art.
I haven't seen you for at least 3 years, so you must have your PHD in Cave Radio by now? :)
 
Dave H said:
I haven't seen you for at least 3 years, so you must have your PHD in Cave Radio by now?

Yes, finished it in Feb 2003. It might be the only PhD in thewotld specifically on "cave radio", although I know of a caver in spain who is currently a PhD student on a similar topic. Im now working in the research department of the UK's Mines Rescue Service, which is probably as close to a job in cave radio as Im likely to get.
 
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