[TowerTalk] Rooftop vertical grounding [SUMMARY]
Guy Olinger, K2AV
Fri, 11 Feb 2000 13:31:49 -0500
I think this is an excellent decision.
Your antenna's performance will depend MORE THAN ANTHING ELSE on the
quality of your ground screen (radials). You may find that the more
ground screen you have, the worse (or narrower) your SWR gets. DON'T
be fooled by that. Monkey with antenna settings afterward, or use a
A properly matched 100% efficient short vertical will ALWAYS exhibit a
narrower SWR curve than a properly matched 100% efficient full-size
1/4 wave vertical. The way one broadens the 80 meter SWR curve on a
short vertical is to make it LOSSY. Obviously counterproductive.
A ground rod in the center of and tied to the radials is sufficient
FOR THE ANTENNA. Do the other stuff you mention as well
**DO** place a good current choke in the feedline at the point where
the coax connects to the vertical. This accomplishes two important
1) On receive, this will help keep misc household noise from getting
into your vertical using the coax shield. Noise can come out from the
house toward the antenna over the coax shield and apply itself over
the ground screen, thereby coupling into the signal path.
2) On transmit, it will prevent your feedline's shield from being used
as part of the antenna. Depending on the particulars of your
installation, it is possible at some frequency(ies) for the shield of
your feedline to exhibit a surge impedance as low as or lower than the
ground screen. The RF will see the feedline as being connected to and
in parallel with the ground screen. The current will divide up with
the greater current on the device with the lower surge impedance. For
example, if they are equal, half of the feed current will flow back
toward the house on the outside of the coax. This in turn is
responsible for at least three undesirables:
A) brings RF back into the house where it can run around on all the
ground connections and cause RFI.
B) changes the feed impedance of the antenna.
C) effectively reduces the efficiency of the antenna. The stuff
radiated in the house can be considered a "loss" since it's not going
where you want it.
On Fri, 11 Feb 2000 09:42:38 -0700, you wrote:
>Thanks to everyone for all the good advice. I've decided that
>the incremental performance gain (if any) from a rooftop installation
>is not worth the risks I'd be exposing the structure and occupants
>to in the case of a direct strike. Even with the proper grounding,
>I would still be worried. Also much easier to for maintenance
>So instead I'm going to ground mount the Butternut in the middle
>of my backyard with a buried radial system.
>Which brings up another question. Would a single ground rod
>tied into the buried radial system be good enough as far as the
>antenna goes? I would still tie into the service ground rod and
>there would be another ground rod near the shack which the
>lightning arrestor would be attached to.
>Thanks again everyone. This people on this mailing list are a
Guy Olinger, K2AV
Apex, NC, USA
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