John Smith wrote:
his secret
location in NV.

9041 Desert Lane
Pahrump, NV 89048

http://maps.google.com

"Roy Lewallen" wrote in message
treetonline...
Here's why antenna efficiency is important for transmitting but not for HF
receiving.

First, the definition of efficiency: For a transmitting antenna, it's the
fraction of the power applied which is radiated. The remainder is turned
into heat. For receiving, it's the ratio of the power which is delivered
to the receiver to the power which could be delivered to the receiver if
the antenna had no loss. The efficiency of a given antenna is the same
when transmitting and receiving. Sometimes people use "efficiency" to mean
other things -- this is the meaning of the term in all antenna literature
and texts.

Consider this communications system:

transmitter - antenna - propagation path - antenna - receiver - listener

A receiver unavoidably adds noise to the received signal. So if no noise
is injected in the propagation path, the signal/noise ratio is the ratio
of the signal entering the receiver to the noise created by the receiver's
input circuitry. This is generally the case at VHF and above.

When receiver noise dominates, as above, increasing the receive antenna's
efficiency increases the signal arriving at the receiver, so the
signal/noise ratio improves. This allows you to hear the signal better.
But it only works for VHF and above.

HF is a different story. At HF, there's a lot of atmospheric noise
(injected in the "propagation path" part of the system), and unless the
receive antenna and receiver are exceptionally bad, the atmospheric noise
is much greater than the noise created by the receiver. I mentioned a
simple test in my last posting, to see whether this is the case -- just
disconnect the antenna. If the noise level drops, atmospheric noise
dominates. It's not hard to make a receiver that atmospheric noise will
dominate with a 3 foot whip antenna at HF. So at HF where atmospheric
noise dominates, the signal/noise ratio is the ratio of the signal
entering the receiver to the atmospheric noise entering the receiver.
Compare this to the situation described above for higher frequencies.

Now let's see what happens when we improve the efficiency of an HF
receiving antenna. Because both the signal and the dominant noise come
from locations in front of (that is, on the transmit side of) the antenna,
improving the efficiency of the antenna makes both the signal and noise
greater in the same proportion when they arrive at the receiver. There's
no improvement at all in the signal/noise ratio. The effect is the same as
turning up the receiver volume control. The only way you can improve the
signal/noise ratio is to somehow favor one over the other, such as by
making the antenna directional. And an inefficient, directional antenna
like a Beverage or small loop will nearly always enable you to hear better
in some directions than an efficient, nondirectional antenna because
directionality helps and inefficiency doesn't hurt.

How about transmit antenna efficiency?

The signal strength from the transmit antenna is proportional to the
antenna's efficiency. (It also depends on other things, but I'm just
talking about efficiency here.) So if the efficiency of the transmit
antenna increases from, say, 33% to 66%, the power levels of the signals
at the receive antenna and the receiver double, and there's no change to
the received noise, on either HF or VHF and above. So improving the
transmit antenna efficiency always improves the signal/noise ratio at the
receiver, in this case by 3 dB.

That's why you can hear bunches of HF stations with a very inefficient
antenna, but they won't hear you if you try to transmit using that same
antenna -- it's because the noise is injected into the system between you.
And it's likely that you'll be able to hear stations just as well with the
very inefficient antenna as with a much larger, efficient one.

Roy Lewallen, W7EL

Well said Roy, however, can you explain why this is not so at VHF and above?
I would think that would have to do more with mode than with
antenna/propagation. When I turn down the squelch on my vhf rx i get lots
of noise. Let me know.
TIA,
B

Brian Oakley wrote:

Well said Roy, however, can you explain why this is not so at VHF and
above? I would think that would have to do more with mode than with
antenna/propagation. When I turn down the squelch on my vhf rx i get
lots of noise. Let me know.
TIA,
B

It's purely because of where the dominant noise comes from, more
specifically whether it gets into the system before or after the
antenna. Atmospheric noise gets greater as you go down in frequency. At
VHF and above, it's less than receiver noise, so receiver noise
dominates and masks whatever atmospheric noise there might be. At HF and
below, it's usually greater than receiver noise, so atmospheric noise
masks the receiver noise. Obviously there's no precise line, so
somewhere typically near the upper end of HF either one might dominate,
depending on conditions, antenna, and receiver.

The noise you get from your VHF radio when you turn down the squelch is
receiver noise. You can prove it by disconnecting the antenna and
noticing that the noise doesn't change. Disconnect the antenna from an
HF receiver and the noise will drop, because it's coming from the other
side of the antenna.

Roy Lewallen, W7EL

On Mon, 29 Dec 2008 20:32:58 -0800, Roy Lewallen
wrote:

It's purely because of where the dominant noise comes from, more
specifically whether it gets into the system before or after the
antenna. Atmospheric noise gets greater as you go down in frequency. At
VHF and above, it's less than receiver noise, so receiver noise
dominates and masks whatever atmospheric noise there might be. At HF and
below, it's usually greater than receiver noise, so atmospheric noise
masks the receiver noise. Obviously there's no precise line, so
somewhere typically near the upper end of HF either one might dominate,
depending on conditions, antenna, and receiver.
(...)
Roy Lewallen, W7EL

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

wrote:

http://www.wellbrook.uk.com/ALA100b.html
With a bit of hacking, the Wellbrook ALA 100 is as good as any of the
shortwave antennas they sell. It is just the amplifier. You have to
roll your own loop. The ALA100 is the lowest cost welbrook. At the
current exchange rate, the ala100 is a over $200.

I have made a few loops with this amp. I have a 2ftx2ft out of copper
pipe for direction finding. I have 4ft x 6ft copper pipe for regular
use. It's really stupid big and will eventually be reduced to the
original 4ftx4ft. I have a few portable designs that are around 40ft
worth of wire.

The wellbrook loops are just amazing. All that said, the ATS909
probably can't handle that much signal. It would make sense to use
one with a portable shortwave radio.

Has anyone ever successfully reverse engineered the pre-amp of a
Wellbrook loop? It might be possible to dissolve the encapsulating
material (epoxy?) to reveal the PCB and componants. Otherwise a medical
scanner (seriously) might reveal some useful information.

Dave wrote:
RHF wrote:

Dave here is a Picture of a . . .
5/8 WL Ground Plane Antenna

No it isn't.

http://users.belgacom.net/hamradio/s...calantenna.htm

"The "ringostar" based coil:
Is made out of 26 cm of 2,5mm installation wire. Remove the isolation of
the wire and tin with a soldering iron the entire wire. The coil is 1,2
turns and has a diameter of 5 cm. One side is connected to the antenna
and the other side to the boom."
.
Note "Installation Instructions" of 5/8 wave vs 1/4 wave antennas. 1/4
wave verticals require a proper ground plane (radials or sheet metal) to
approximate the other half of a center fed dipole. The 5/8 wave is
already over a half-wave long; no plane required. The coax shield
needs a ground, the antenna doesn't.

http://www.diamondantenna.net/m285.html

http://www.diamondantenna.net/hf6fx.html

You can use the outer-braid of the coax, or a ground wire, as a
counterpoise, as your text indicates ... but, for proper operation, at
least in all my experience, a counterpoise IS necessary.

However, as is common, people claim poor antennas are more than
satisfactory, for them ...

Regards,
JS

flashdrive wrote:

...
Has anyone ever successfully reverse engineered the pre-amp of a
Wellbrook loop? It might be possible to dissolve the encapsulating
material (epoxy?) to reveal the PCB and componants. Otherwise a medical
scanner (seriously) might reveal some useful information.

My question would be, "Why go to the trouble?"

Indeed, grab a DC - 1Ghz MMIC device (make sure you don't get an SMC
component, unless you like soldering under a microscope), stick a proper
filter for the freqs/bands in front of it, and feed its' input with a
well designed loop ... if you need EXTREME gain, you can cascade a
couple of MMICs.

Regards,
JS

Jeff Liebermann wrote:

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.

In my last couple of postings, I was lumping man-made and atmospheric
noise together as "atmospheric noise". Both enter the system between the
transmit and receive antenna, so improving the receive antenna
efficiency won't help the ratio of signal to either atmospheric or man
made noise. The referenced graph doesn't show receiver noise at all,
which dominates at VHF and above.

It can be useful, however, to distinguish between atmospheric noise and
*local* man-made noise, since the latter can sometimes be reduced by
using techniques such as feedline decoupling and using horizontally
polarized antennas.

Roy Lewallen, W7EL

In article
,
Telamon wrote:

In article ,
Dave wrote:

John Smith wrote:

However, in side-by-side comparisons on 10-6-2m antennas I have built,
comparing a 5/8 against the 1/2 (construction methods/materials and
matching components identical) ... the actual difference, in the real
world, must be less than the width of a meter needle in the readings ...
or, put simply, I no longer deal with the extra length required of the
5/8 ... your mileage may vary ...


The advantage of a physical height (antenna length) between 180 and 215
degrees (see previous post regarding the magic number being around 195
degrees) is improved take-off angle and reduced skywave-groundwave
interaction, not dramatic nearfield voltage increases.

Regarding Mr. Smith's comments above my experience and others is the
opposite. 5/8 is a much better performing antenna than a 1/2 wave for
local VHF and UHF communications. Well worth the effort to build a 5/8
wave antenna over a 1/2 wave. The 5/8 had some kind of series load coil
part way up the whip where the 1/2 wave match/compensation was done at
the base so the whip was solid. Sorry I can't more specific then that as
those experiments were many years ago.

Mr. Smith is still lost in space.

Here is an example of the 5/8 wavelength antenna I recall using in the
center of the page. The one I used was permanent mount not magnetic
though. The van roof it was installed on was the ground plane.

http://www.new-tronics.com/main/html/mobile_vhf.html

When this antenna was changed for a 1/2 wave a lot of coverage was lost.
This was before cell phones so I had to start using pay phones a lot.

--
Telamon
Ventura, California

John Smith wrote:
Dave wrote:
RHF wrote:

Dave here is a Picture of a . . .
5/8 WL Ground Plane Antenna

No it isn't.

http://users.belgacom.net/hamradio/s...calantenna.htm

"The "ringostar" based coil:
Is made out of 26 cm of 2,5mm installation wire. Remove the isolation
of the wire and tin with a soldering iron the entire wire. The coil is
1,2 turns and has a diameter of 5 cm. One side is connected to the
antenna and the other side to the boom."
.
Note "Installation Instructions" of 5/8 wave vs 1/4 wave antennas.
1/4 wave verticals require a proper ground plane (radials or sheet
metal) to approximate the other half of a center fed dipole. The 5/8
wave is already over a half-wave long; no plane required. The coax
shield needs a ground, the antenna doesn't.

http://www.diamondantenna.net/m285.html

http://www.diamondantenna.net/hf6fx.html

You can use the outer-braid of the coax, or a ground wire, as a
counterpoise, as your text indicates ... but, for proper operation, at
least in all my experience, a counterpoise IS necessary.

However, as is common, people claim poor antennas are more than
satisfactory, for them ...

Regards,
JS

A "counterpoise" is not a ground plane.

We all would like to drill a hole in the middle of the roof, but
sometimes we are forced to clamp to something. The 5/8 Wave works well
on a clamp, at least as well as the 1/4 Wave in the middle of the roof.

"...However, as is common, people claim poor antennas are more than
satisfactory, for them ..."

"Buckets of irony littered the lobby"