Hi Bob,

Ground losses aren't extremely important for receiving applications. A
few extra feet of depth of ground rod isn't going to make any
difference, because the RF doesn't flow 8 feet down, or 5 feet down,
but more flows on the surface of the ground and/or some inches down
(depending on your ground conductivity profile)

As such, you could try a 4 or 5 foot ground rod.

If you want a no-pounding solution instead, lay down a few radial
wires. The length and number aren't critical. Try four or eight wires
each 1/8 wavelength long at the lowest frequency of interest.

If the noise in your receiver doesn't increase when you plug the
antenna into the RX , then you may want a better ground system. If the
noise DOES increase, then your noise floor is that of the
natural/artificial noise that your antenna is picking up, and more raw
signal from the antenna won't help anything. (Though moving the
antenna around might).

73,
Dan
N3OX
www.n3ox.net

I don't know that I buy your statement that ground losses aren't
extremely important for receiving applications. Antenna systems are
bi-directional. If you lose xdB of transmitted sigal due to ground
losses, then you are also going to lose xdB of received signal due to
ground losses.

A more important issue is that ground losses are very important when
receiving a lightning strike. It may be effective to bury several
runs of bare #6 copper in trenches radiating from your "ground
point". The QTH here is located on a solid rock slab. All grounding
is through buried #6 wires and a few ground rods driven laterally
between the rock layers up by the tower base.

Regards,
Ed

On 11 Apr 2006 12:46:24 -0700, "
wrote:

Hi Bob,

Ground losses aren't extremely important for receiving applications. A
few extra feet of depth of ground rod isn't going to make any
difference, because the RF doesn't flow 8 feet down, or 5 feet down,
but more flows on the surface of the ground and/or some inches down
(depending on your ground conductivity profile)

As such, you could try a 4 or 5 foot ground rod.

If you want a no-pounding solution instead, lay down a few radial
wires. The length and number aren't critical. Try four or eight wires
each 1/8 wavelength long at the lowest frequency of interest.

If the noise in your receiver doesn't increase when you plug the
antenna into the RX , then you may want a better ground system. If the
noise DOES increase, then your noise floor is that of the
natural/artificial noise that your antenna is picking up, and more raw
signal from the antenna won't help anything. (Though moving the
antenna around might).

73,
Dan
N3OX
www.n3ox.net

Ed Bailen wrote:
I don't know that I buy your statement that ground losses aren't
extremely important for receiving applications. Antenna systems are
bi-directional. If you lose xdB of transmitted sigal due to ground
losses, then you are also going to lose xdB of received signal due to
ground losses.

Your statement about the "bi-directionality" (usually called
reciprocity) of antennas is true. But what counts when receiving is the
signal to noise ratio. If you lose x dB at the transmitter, the person
receiving the signal gets a signal that's now x dB lower than it was
before, and the noise is the same. So the S/N ratio has been reduced by
x dB. Therefore it's desirable to minimize loss at the transmitter antenna.

But what happens when the receive antenna has x dB loss? At HF, the
dominant source of noise is atmospheric (and QRM). Adding x dB loss at
the receive antenna reduces both the signal *and* the noise by x dB. The
result is the same S/N ratio as before. That's why efficiency isn't
important for HF receiving antennas. Of course, you could reach a point
where the efficiency is so bad that the receiver noise dominates. Beyond
that point, lowering receive antenna efficiency will reduce S/N ratio.
But this point is usually a long way down. Likewise, at VHF and above,
where atmospheric noise is low and receiver noise dominates, high
receive antenna efficiency is desirable. But the original question was,
I believe, in reference to HF receiving.

A more important issue is that ground losses are very important when
receiving a lightning strike. It may be effective to bury several
runs of bare #6 copper in trenches radiating from your "ground
point". The QTH here is located on a solid rock slab. All grounding
is through buried #6 wires and a few ground rods driven laterally
between the rock layers up by the tower base.

Lightning protection and mains safety grounding are separate issues with
somewhat different requirements. Although a good lightning ground
probably usually constitutes a reasonably efficient RF ground, that's
not always the case, and the reverse isn't necessarily true either.

Roy Lewallen, W7EL

I disagree about the penetration depth. The depth depnds on frequency and
soil characteristics, but at 160 m the "skin depth" (the point where the
potential has fallen to 1/e of its original value) would normally fall in
the range of 10 - 30 feet.

If you live over a silver mine or the Sahara desert, your mileage will vary.


Chuck

I stand corrected!

I should have calculated it.

Sorry for including gross misinformation about MF & HF skin depth in
typical soil.

A modest radial system will whip a deep ground rod, though. I'm about
10% tempted to break out J.D. Jackson' Classical Electrodynamics and
try to calculate the RF resistance vs. depth of a ground rod, but I
know it's going to be hard to deadly.

Quantitative is always best in this sort of discussion, for sure.

Thanks for the response.

Dan,
N3OX

wrote:
. . .
A modest radial system will whip a deep ground rod, though. I'm about
10% tempted to break out J.D. Jackson' Classical Electrodynamics and
try to calculate the RF resistance vs. depth of a ground rod, but I
know it's going to be hard to deadly.
. . .

If you decide to, I'll be interested in seeing how you do it and what
results you get. Reg is the only one I've seen so far who can come up
with the impedance of a one-terminal device.

For those of us stuck with needing two terminals to determine a
resistance, we find that the RF resistance of a ground rod depends on
what's above the ground. That is, the RF resistance of the ground rod is
a function of the nature of the antenna. For example, here's the
resistance of a four foot ground rod in average soil at 14 MHz, beneath
vertical antennas of various heights:

Ant ht ft Ground rod R ohms
16.8 46.2
8.4 34.8
4.2 18.3

These were determined by using NEC-4 and comparing the feedpoint
resistance of the vertical over perfect ground with the resistance when
connected to a ground rod in average ground. I assume the change in
apparent ground rod resistance is due to the change in the field in the
ground from the antenna as the antenna height varies.

Quantitative is always best in this sort of discussion, for sure.

Indeed.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Reg is the only one I've seen so far who can come up
with the impedance of a one-terminal device.

How many terminals are there for an EM wave traveling
through empty space?
--
73, Cecil http://www.qsl.net/w5dxp

Right, the question of what the RF resistance of a ground rod doesn't
make sense without the antenna.

What happens if you change the ground rod length to 8 feet and feed
each of those antennas against it?

Dan,
N3OX