Re: NEC-LIST: Small antennas: can they me multiply resonant?

From: D. B. Miron <dbmiron_at_email.domain.hidden>
Date: Wed, 8 Jan 2003 08:02:07 -0600

Alan,

No, I haven't seen any valid gain measurements published or otherwise for
either CTHA or fractal antennas. I'm sure there aren't any for the CTHA,
because no one built one that follows my recommendations. However, I was
not aware of a commercial HF version. Could you e-mail the article or
provide a web link? If not, the reference might help. There are also no
gain measurements on a volume-loaded dipole, although it would be surprising
if the gain were different from a simple small dipole of the same size. The
issue with the simple dipole isn't it's own copper loss, it's the loss in
the required tuning inductor.

I have designed an HF band volume-loaded vertical dipole, 4 m tall by 4 m
wide. I built it with 0.5" copper tubing for the volumes, and it has two
radiating elements that are switched in as needed. One is a coil of 0.25"
i.d. soft copper for the 80 m band, and the other is a straight piece of
0.5" copper for 40 m and up. The interesting thing about this application
is that reasonably conductive ground raises the input resistance to 12-14
ohms at 4 MHz. This effect was predicted by NEC2 and verified by my
measurements. I've had problems with my transceiver and power supply, so
I've only been on the air briefly since I built it last Summer. Just
yesterday I went out to my shop which is also now my radio room and gym,
reconfigured the equipment with a new power supply only to discover that the
remote tuner seems to be dead. Eventually I hope to get a reliable system
working and do some comparisons with the local ham club.

Doug Miron
----- Original Message -----
From: <alan.boswell_at_baesystems.com>
To: <dbmiron_at_paulbunyan.net>
Sent: Tuesday, January 07, 2003 9:49 AM
Subject: RE: NEC-LIST: Small antennas: can they me multiply resonant?

Doug

Thank for that and I have seen those papers. Volume loading look like a
neat way of using the Chu sphere efficiently. The CTHA antenna has been
around for a while and your analysis looks right, but the antenna itself
seems to have been disappointing in its performance according to a review
published a while ago of a commercial HF version, which did simple
comparisons with dipole antennas. The CTHA seems to have been about 4 'S'
points down on 3.5MHz, reducing to 2 'S' points on 14MHz (1 S-point being
4-6dB). I do not know of any accurate and traceable gain measurements for a
CTHA - have you seen any? or any for fractal antennas?

Best regards,

Alan

-----Original Message-----
From: D. B. Miron [mailto:dbmiron_at_paulbunyan.net]
Sent: 07 January 2003 15:02
To: alan.boswell_at_baesystems.com
Cc: NEC-LIST
Subject: Re: NEC-LIST: Small antennas: can they me multiply resonant?

Good day Alan,

Here are a couple of papers I've written. Volume loading is a way of
getting so much structural capacitance that series resonance can be induced
even for very tiny antennas. Of course, high Q becomes an issue if you go
small enough. The CTHA is essentially a radiating transmission-line
structure which is electrically-small at the first three standing-wave
resonances.

"Volume Loading-A New Principle for Small Antennas", ACES Journal, July '99.

"A Study of the CTHA Based on Analytical Models", IEEE Trans. on Antennas
and Propagation, August, 2001.

Also, there are fractal loops that have resonances below 0.3 wavelength. I
can't find the last article I saw on the subject, you might contact Chip at
fractenna_at_aol.com.

----- Original Message -----
From: <alan.boswell_at_baesystems.com>
To: <dbmiron_at_paulbunyan.net>; <nec-list_at_gweep.ca>
Sent: Tuesday, January 07, 2003 4:53 AM
Subject: RE: NEC-LIST: Small antennas: can they me multiply resonant?

Doug

I am interested in who has found such structures and where the results were
published. An electric element has a small radiation resistance in series
with a capacitive reactance, an magnetic element is the same except the
reactance is inductive. What other sorts are there?

All the best,

Alan

-----Original Message-----
From: D. B. Miron [mailto:dbmiron_at_paulbunyan.net]
Sent: 06 January 2003 19:16
To: NEC-LIST
Subject: Re: NEC-LIST: Small antennas: can they me multiply resonant?

David, Steve, and Alan,

I think most of us working on small antennas define "small" in terms of
wavelength of operation, rather than being subresonant. Some of us have
found structures that don't include lumped elements that have one or more
resonances at wavelengths where the antenna is still small. So, I agree
with Steve's posting.

Doug Miron

----- Original Message -----
From: <alan.boswell_at_baesystems.com>
To: <d.jefferies_at_surrey.ac.uk>; <nec-list_at_gweep.ca>
Sent: Monday, January 06, 2003 11:49 AM
Subject: RE: NEC-LIST: Small antennas: can they me multiply resonant?

David
If you define a small antenna as an antenna operated below its lowest
natural resonant frequency, the question disappears (lumped components must
be disallowed).
Alan

-----Original Message-----
From: David Jefferies [mailto:d.jefferies_at_surrey.ac.uk]
Sent: 06 January 2003 17:02
To: nec-list_at_gweep.ca
Subject: NEC-LIST: Small antennas: can they me multiply resonant?

Dear List,

Revisiting the arguments about the CHU limit on Q for
small (very small) antenna structures, in the light of
my comments about multiple resonances, can a structure
which is very much smaller than the radian sphere
contain multiple radiating resonant elements, or is
it necessarily the case that small antennas are only
singly-resonant?

regards

--
David Jefferies
http://www.ee.surrey.ac.uk/Personal/D.Jefferies/
http://www.eryptick.net/
fax +44 (0)1483 534139 attn D.Jefferies
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Received on Wed Jan 08 2003 - 13:59:18 EST

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