T&T: Copper pipe for earth connections in SSB radio

[jason_kovatch at agilent.com]

Hello Jim,

If you check the web for experienced reports of marine SSB installations you
will find that most all suggest a ground strap (not foil) at least 2-3" wide.
I would go with that. I am not a marine SSB expert. My main experience with HF
RF is terrestrial ham operation. (I will be installing my maritime mobile HF
rig this summer. I plan to use strap.)

Here's the deal: Radio Frequency energy (RF) is funny stuff; that is, it is
unlike ordinary electricity as it has a very high AC frequency component,
millions of cycles per second, as opposed to AC wall current at 60 cycles. For
DC the pipe and the strap of equivalent cross-sectional area would be
indistinguishable in practice. For RF it is different. The AC component makes
the energy reluctant to travel down a conductor that appears too constricted
(technically this is called a reactive impedance mismatch, which varies with
frequency). The surface area of the grounding conductor (and evidently the
shape) makes a big difference to the impedance at HF frequencies. RF will flow
on both sides of a strap, but will not flow on the inside of a pipe, so it
would take a very large pipe to equal the effective surface area of a 3"
strap, like maybe a 2 inch pipe.

Remember that the energy produced by the transmitter is not just going out the
antenna- the ground conductor is the other half of the transmission line and
carries current back and forth to the transmitter. If the antenna is working
right, the energy is coupled to the air at the end of the antenna. Since the
grounding conductor is also conducting every cycle of AC, it needs to conduct
to "ground" also. The 3" strap recommendation is the result of many
installations that work well. What happens when the RF is reluctant to travel
down the conductor is it "stands" around, and can actually burn you when
transmitting if it can't get away. Shocks on the microphone to lips or hands,
especially on corners of the transmitter are signs of inadequate grounding. I
have experienced this with grounding that I thought was adequate, and seen it
go away when copper ground wires were replaced with 2" copper braid, which
works better than strap because of the extra surface area of all the
individual conductors, but it degrades rapidly in a salt environment.

Getting a good ground system is probably the most common reason for poor HF
performance, I think. I would do everything I could to make it as good as
possible (in this case electrically "big"). On a side note, if you do decide
to use pipe, drill some holes to allow water to drain out. I think strap (not
bar, more like a flashing thickness) would be a lot easier than flattened or
round pipe to install, as it is easily bendable.  Just nail it down wherever
you were going to mount a pipe clamp, and paint it.

This all supposes you have a great "ground" somehow that will accept all the
energy you finally conducted. Using screen to couple to the water through the
hull is another approach that "looks" pretty good to HF, but again the area
needs to be pretty big to look inviting to RF.

Part of the problem is figuring out how well an antenna system is working.
Measuring the "SWR" of the antenna will give a big clue. You are looking for a
system SWR of less than 2:1 ("2 to 1", with 1:1 being a perfect match.) I
think it is indispensable for those working with HF to measure SWR and have
meter to check it. Make sure it will measure down to 30 MHz, not just CB. They
can be inexpensive, here is one from Radio Shack for $50:

http://www.radioshack.com/product/index.jsp?productId=2036239

The only problem with measuring SWR is that even perfect SWR does not tell you
if the antenna is radiating in a good pattern. A light bulb can have a good
SWR but makes a poor antenna. But you need good SWR regardless. The energy has
to get out to the antenna and into the air.

(the link below is a good example of a good grounding system, with diagrams-no
connection to me)
http://www.marinenet.net/antenna%20info.htm

Here is an article that describes making the RF ground NOT conductive at DC,
to avoid corruption of the galvanic protection:

http://sailmail.com/grounds.htm

I would be interested in hearing from anyone who has gone to the trouble of
installing screen for a ground system for HF, and how it works.

Jason Kovatch
CHB34 Bay Heart
Everett Wa
NJ7K

-----Original Message-----
   Message: 10
Date: Sun, 1 Feb 2009 13:19:49 -0500
From: "Jim Healy" <gilwellbear at gmail.com>
Subject: T&T: SSB/Ham ground plane on a boat
To: <trawlers-and-trawlering at lists.samurai.com>
Message-ID: <573F29579B25463BA4FC6F0EEE82AAE0 at JBHA215>
Content-Type: text/plain; charset="us-ascii"

Folks,

Question: relative to Marine SSB and Ham radio transceiver applications,
this morning I had a conversation with a boater who is planning to build an
RF ground plane out of 3/4" copper tubing, like you'd find in a house
plumbing system, instead of copper foil.  His reason was that pipe is much
easier to handle and secure during installation in those parts of the bilge
where humans don't fit, and of course, it's much more durable than foil.
Since RF flows on the surface of a ground plane conductor, and the
circumference of 3/4" pipe approximates the 3" that is commonly found on
commercial foil rolls, it seemed reasonable to me.  Do any of you have
experience with this approach?  Does it work?  Does it work well (low SWR)?
Does this seem to be a practical and reasonable idea?  Second question:
electrically, does a ground plane need to be end fed, or does it matter
where the feed point is?

Peg and Jim Healy aboard Sanctuary,
currently at Charlotte Harbor, Punta Gorda, FL
AGLCA # 3767
MTOA # 3436