K9OM posted:
>I'd appreciate your thoughts on a RF problem at my summer QTH: A tribander is
>mounted on a self-supporting 50' tower. (no guy wires) The tower is mounted
>3' away from the shack... which is a 64' mobile home. Any power level over
>75 watts usually causes the rig to lock up at full power. (whether on 10,15,
>or 20 m.) Various types of grounding have been tried without success. At
>this
>time, the tower is grounded with 8' ground rods and #2 copper wire going
>from the tower to the outside electric main entrance box which is 14'
>away. The
>frame of the mobile home is also grounded to this same common ground. (I've
>tried the frame grounded and ungrounded with no difference)
>
>I am thinking there must be interaction between the antenna and the steel
>frame under the mobile home. The actual length of the steel frame is
>61'. Of
>course, there are two of these 61' I frames under the house.
>
>I have had this problem with 3 different HF radios so it isn't a radio
>problem. Of course the rig is grounded to the common ground mentioned
>above.
>(grounding the rig helped a little but not much). Sometimes I need more
>than 75
>watts... so all ideas are appreciated as to how I can crank up the power
>without the rig going into orbit.
This sounds like my station. "Any power level over 75 watts usually
causes the rig to lock up at full power" - do you mean that talking into
the mic it goes squirrelly on its own after reaching that power level, or
instead that say as you bring the power up when key down tin CW
that it locks key down at that point?
The latter happens to me. All rigs were locking key down as the
RF was getting into what was keying the rig (both key line & PTT).
The culprit was stuff coming down the various lines coming into
the shack (feeders, rotor cable, etc). I wish I had enough length
to also coil everything just before it enters the shack, but most of
my problems went away after coiling the lines at the base of the
tower in order to series feed it as an LF vertical.
Instead of grounding, look at decoupling - part of my problem was
also lack of decoupling of lines going into device keying the radios
(as in not even bypass capacitors across its external connections).
Only by choking off the RF coming down the cables plus beefing
up the boxes connected to the radios was I able to stop things locking
up with sometimes far less that 400 watts into an antenna about 7m &
one slab of somewhat reinforced concrete above the shack.
73, VR2BrettGraham
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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