One of the issues I see over and over is the lack of understanding about
grounds. Somewhere along the line of ham radio linage someone once said
that we must have a ground for the station to operate correctly. That
really should be restated to say "a ground is needed for the antenna to be
an efficient radiator". That is correct as applied to un-balanced antennas
such as end fed wires, off center fed Zepps and Windom's verticals and the
like. As to station ground, meaning the grounding of ones radio, it really
is not necessary except for safety reasons.
A dipole fed direct with coax having a feedline length of over 1/4 wave is
really quite balanced and a station ground or antenna ground is not needed.
In some cases the addition of a ground wire from the station to the earth
can upset the electrical length to the extent the radio then is factually
above RF ground.
In actuality, we often think that running a good size conductor from the
radio to the driven ground outside of the window or wall is a good ground.
From a viewpoint of DC ground it is. From a viewpoint of RF ground it can
be a very poor ground to the extent of being actually harmful. Take a case
where the ground wire is 16 ft in length. That is about 1/4 wavelength or
90 degrees electrically on 20M. If one end is "ground" then the other end
is likely at maximum RF potential and on 40M it is 45 degrees electrically
above ground.
I have always viewed and approached grounds in three ways: (a) ground for
an antenna system which is required to operate against earth for efficient
radiation. AM broadcast station verticals are great examples of this need.
(b) ground for personnel safety reasons. This is no more than the 3rd pin
AC ground back to the mains box or neutral leg of the mains. (c) lightning
ground for the protection of the structure and the associated equipment.
In each case a clear separate method and approach is required. (b) is easy.
Just don't cut or lift the 3rd pin on any equipment and be sure the 3rd pin
on the receptacle is connected back to the ground buss at the AC mains
panel. The NEC requires a driven ground at or below the main service
entrance. (a) is always outside of the house and usually consists of
radials and driven rods all connected back to a common point for the antenna
feed. In broadcast applications this is usually at the base of the tower.
It is then normal to bring a common ground back into the transmitter
building. (c) is the point most overlooked, mis-understood and most often
very improperly done to the extent of presenting a dangerous condition.
Actually (c) is easy to implement in as much as all driven grounds for a
site supplied by the AC mains must be bonded or connected together. Yes,
every ground rod, ground system, AC mains ground rod, telephone ground rod,
cable TV ground rod, sat TV antenna ground rod , well pump, radio tower
ground or mast and such must be connected together outside of the structure
with a #6 or larger conductor. It is not rocket science, it is plain simple
Ohms Law being applied. E = I x R Take any two earth driven grounds and
you will find there is some value of resistance between them. Take a value
of current dissipated through the earth due to a nearby lightning strike.
Do the math and you have a value of E which is the voltage between the two
driven grounds. Is your radio connected between these two grounds? If so,
the path of least resistance is through your radio equipment. To work an
example: R = 5 ohms {actually this is low for most applications}, I = 1000
amps {lightning dissipated currents can be much higher} and thus E = 5000
volts. YIKES, this can be the spike potential that flows through your radio
and power supply.
If we insist on using antennas that are unbalanced then some provision must
be made to prevent RF from getting back to the station equipment. The word
"RF Choke" comes to mind. The least expensive, most efficient means,
although ugly, is no more than 15 to 20 turns of coax wound in a single
layer around a piece of 3"OD PVC pipe. An alternative more modern approach
is to put sever ferrite donuts over a piece of coax. In one book these are
referred to UnUns. Unbalanced to Unbalanced devices. Most likely this
should be used between the tuner and the radio.
It is springtime folks and with springtime come thunder storms containing
lots of lightning. Check your ground system integrity to make sure it is
correct and safe. And if you've done it correct, most of the RF problems
will no longer be of concern. As to antenna radiation back into the shack,
that's a topic for another discussion. Hint: space helps.
73
Bob, K4TAX
One thing I would add: I found RF getting into me O2 primarily through
the
headphone connector and the CW paddles connector on the back panel.
Ferrites on each fixed the problem I had no problem with RF on the mic,
although the input to the W2IHY 8-Band did have a problem on 10 meters
when
I cut the cable to the right length for the shack, which turned out to be
8'...a nice quarter-wave on 10! Duh! A ferrite fixed that, too.
I should by stock in Amidon....
Thanks to all for your suggestions. It seems that this is a fairly
common
issue. Here are the things I've gleaned.
1) Antenna, albeit resonant, being so close to the shack (20 feet
overhead) may be apt to coupling to ground in the rain.
2) Toroids, Toroids, Toroids. At least 1 for each input to the rig,
including on the power leads.
3) I need to install an earth ground, and potentially share it with my
satellite TV's ground.
4) TenTec needs to seriously look at this issue, because it sounds like
it's a VERY common one and maybe indicative of a design flaw.
In regards to number 4, I still love this rig. I will not be giving it
up any time soon. Just need to make some mods to the way things are set
up.
>> I had a problem with my O1 & O2 with the TT 706 mic. Key stuck and RF
>> in
the
audio. I found that when I pointed the beam right over the shack the
problems started. I was able to eliminate almost all the problems by
taking
the 706 mic apart and using kitchen foil placing it around the little PC
board pre amp and leaving a tab to put under one of the bottoming
mounting
screws, that grounded the foil. I first put black tape on the board to
insulate it from the foil. Yes, the shack is grounded to a common 2, 8
foot
ground rods 5 feet apart.
I also found last year when I had the key stuck problem it turned out to
be
the PTT button switch. It was bad. Had to bang on it to get back to
receive.
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