All,

I am interested in experimenting with ferrite core receive antennas. I
have about a dozen .4 by 10 inch rods. The primary band of interest is
75m. My question is:

1. For best reception is it better to make a fat or a long antenna? I
cannot find much written about these antennas and have not found any
equations.

The literature states the efficiency of these antennas can range from a
10**3 to 10**6 a dipole of the same size. Let's not argue the high end
but accept the low end for discussion. Does this ratio refer to length
or diameter?

Is there a good primer source? I have the ARRL Antenna handbook.

Thanks - Dan

dansawyeror wrote in news:gdKdnbMJd42f-
:

All,

I am interested in experimenting with ferrite core receive antennas. I
have about a dozen .4 by 10 inch rods. The primary band of interest is
75m. My question is:

1. For best reception is it better to make a fat or a long antenna? I
cannot find much written about these antennas and have not found any
equations.


I seem to recall that this is a design problem often treated in texts.
Certainly Kraus has a treatment in his book.

In the Fig 3 in my article
http://www.vk1od.net/SmallUntunedSquareLoop/index.htm , I give the
formulas for calculation of the o/c voltage in a single turn loop in air.
In the case of a ferrite loop, the area will be the csa of the loopstick,
and you will have to multiply the single turn value by the number of
turns and include the effect of relative permeability of the ferrite. You
can then proceed to work out the source impedance for the ferrite coil
and the loaded voltage (including the effects of tuning if used).

The literature states the efficiency of these antennas can range from a
10**3 to 10**6 a dipole of the same size. Let's not argue the high end
but accept the low end for discussion. Does this ratio refer to length
or diameter?

Is efficiency a good indicator of the "goodness" of a rx antenna where
the ambient noise is high (as it is on 80m)?

Perhaps if you want to receive normal signals, you just need an antenna
that will deliver 6dB more noise (ie from ambient noise) to the receiver
than its equivalent internal noise. In that case, the received S/N will
not be more than 1dB worse than with a lossless antenna system.

Owen

Owen Duffy wrote:
Perhaps if you want to receive normal signals, you just need an antenna
that will deliver 6dB more noise (ie from ambient noise) to the receiver
than its equivalent internal noise. In that case, the received S/N will
not be more than 1dB worse than with a lossless antenna system.

Owen

Andy writes
A good point..... The rule of thumb I have always used is that if
you turn up the receiver to hear the noise, and THEN connect the
antenna,..... if the noise level goes up a lot, then everything is
working just fine...... At HF, it isn't unusual to have 30uv/m of
"noise", from cosmic or galactic or whatever... A 20 db noise
figure is just fine for a receiver in those ranges...... The biggest
challenge is man-made interference ......

I have seen several articles using ferrite loops at HF, mostly
for building direction finders.... If the ARRL handbook has a
heading under "direction finders", or Hints and Kinks, or
the ARRL Antenna Handbook, they can give the OP some ideas....

Andy W4OAH in Eureka, Texas

"AndyS" wrote in news:1169524019.256493.196630
@a75g2000cwd.googlegroups.com:

working just fine...... At HF, it isn't unusual to have 30uv/m of
"noise", from cosmic or galactic or whatever... A 20 db noise
figure is just fine for a receiver in those ranges...... The biggest
challenge is man-made interference ......


Dan,

Some more information on expected ambient noise per ITU-R p.372-8 is in my
article at http://www.vk1od.net/fsm/FSAmbientNoise.htm . For 3.6MHz, it is
probably wise to expect ambient noise field strength somewhere around
6dBuV/m in 2kHz bandwidth in a residential environment. If an antenna can
develop more than about -130dBm in the receiver from that field strength,
the S/N should be ok on most modern SSB receivers which have a noise floor
somewhere around -136dBm. If you were in a quieter environment, you would
benefit from a better antenna.

Owen

"dansawyeror" wrote in message
...
All,

I am interested in experimenting with ferrite core receive antennas. I
have about a dozen .4 by 10 inch rods. The primary band of interest is
75m. My question is:

1. For best reception is it better to make a fat or a long antenna? I
cannot find much written about these antennas and have not found any
equations.

The only work I've seen done on multiple ferrite rods was for use at
the AM BCB frequency and they bundled them side by side for a short
fat antenna.
IIRC the information was in an NRC (National Radio Club) publication.
I think you will find an index of their publications online.
Mike

Mike

Owen,

Thank you. The environment is medium density residential. I am listening
to a net regularly on 3970kc. I have limited space and cannot rig a half
wave dipole and am looking for alternatives.

Frequently a station close by picks up stations I cannot hear. I am
looking for small space alternatives. Given that that station is in the
same 'cosmic' noise environment what characteristics create a superior
s/n ratio?

For receive only is a ferrite core a good alternative?

- Dan

Owen Duffy wrote:
"AndyS" wrote in news:1169524019.256493.196630
@a75g2000cwd.googlegroups.com:

working just fine...... At HF, it isn't unusual to have 30uv/m of
"noise", from cosmic or galactic or whatever... A 20 db noise
figure is just fine for a receiver in those ranges...... The biggest
challenge is man-made interference ......


Dan,

Some more information on expected ambient noise per ITU-R p.372-8 is in my
article at http://www.vk1od.net/fsm/FSAmbientNoise.htm . For 3.6MHz, it is
probably wise to expect ambient noise field strength somewhere around
6dBuV/m in 2kHz bandwidth in a residential environment. If an antenna can
develop more than about -130dBm in the receiver from that field strength,
the S/N should be ok on most modern SSB receivers which have a noise floor
somewhere around -136dBm. If you were in a quieter environment, you would
benefit from a better antenna.

Owen

Owen,

Thank you. The environment is medium density residential. I am listening
to a net regularly on 3970kc. I have limited space and cannot rig a half
wave dipole and am looking for alternatives.

Frequently a station close by picks up stations I cannot hear. I am
looking for small space alternatives. Given that that station is in the
same 'cosmic' noise environment what characteristics create a superior
s/n ratio?

For receive only is a ferrite core a good alternative?

- Dan

Owen Duffy wrote:
"AndyS" wrote in news:1169524019.256493.196630
@a75g2000cwd.googlegroups.com:

working just fine...... At HF, it isn't unusual to have 30uv/m of
"noise", from cosmic or galactic or whatever... A 20 db noise
figure is just fine for a receiver in those ranges...... The biggest
challenge is man-made interference ......


Dan,

Some more information on expected ambient noise per ITU-R p.372-8 is in my
article at http://www.vk1od.net/fsm/FSAmbientNoise.htm . For 3.6MHz, it is
probably wise to expect ambient noise field strength somewhere around
6dBuV/m in 2kHz bandwidth in a residential environment. If an antenna can
develop more than about -130dBm in the receiver from that field strength,
the S/N should be ok on most modern SSB receivers which have a noise floor
somewhere around -136dBm. If you were in a quieter environment, you would
benefit from a better antenna.

Owen

Owen Duffy wrote:

Dan,

Some more information on expected ambient noise per ITU-R p.372-8 is in my
article at http://www.vk1od.net/fsm/FSAmbientNoise.htm . For 3.6MHz, it is
probably wise to expect ambient noise field strength somewhere around
6dBuV/m in 2kHz bandwidth in a residential environment. If an antenna can
develop more than about -130dBm in the receiver from that field strength,
the S/N should be ok on most modern SSB receivers which have a noise floor
somewhere around -136dBm. If you were in a quieter environment, you would
benefit from a better antenna.

Owen

Andy writes:
Thanks for posting the website, Owen.....
My comments were based on a residential/business site with a bandwidth
that
could accomodate AM, and I was generalizing .....

When I was designing LORAN (100khz) set sfor aircraft (TI2000) , I
needed to
find how small an external whip antenna could be... It turned out that
22
inches was sufficient, since a larger antenna which would convert more
signal, would also convert, proportionately, more noise......

I guess that's why whip antennae on AM car radio are only a couple
feet long, and why loopsticks work so well at AM broadcast.....

A nice article you wrote,.... thanks for giving the web reference...


Andy W4OAH, in Eureka, Texs

dansawyeror wrote in
:

Owen,

Thank you. The environment is medium density residential. I am
listening to a net regularly on 3970kc. I have limited space and
cannot rig a half wave dipole and am looking for alternatives.

Frequently a station close by picks up stations I cannot hear. I am
looking for small space alternatives. Given that that station is in
the same 'cosmic' noise environment what characteristics create a
superior s/n ratio?

If you looked at the reference material I gave you, you will see that you
might expect the ambient noise in a residential environment to develop
about -83dBm in a 2kHz wide rx with a lossless antenna.

If your receiver noise floor was say -136dBm (Noise Figure about 5dB, a
state of the art HF receiver), and you wanted a 6dB margin to restrict
degradation of the "off-air" S/N to 1dB, then your antenna needs to have
gain of at least ( -136 - -83 + 6 )dBi, or -47dBi. That is not very
difficult to achieve, even with a ferrite loop antenna.

Owen

On Tue, 23 Jan 2007 11:12:02 -0800, dansawyeror
wrote:

For receive only is a ferrite core a good alternative?

Hi Dan,

Yes, but only insofar as it is balanced and gives you sharp nulls to
help eliminate some of the interference. This may require elevating
it away from reflecting surfaces if it is not shielded.

73's
Richard Clark, KB7QHC