----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "RFI List" <rfi@contesting.com>
Sent: Wednesday, May 10, 2006 5:49 AM
Subject: Re: [RFI] Defibrillator and RF Exposure Query
>
> Inverse square law, of course, will apply, so any antenna system
> that maximizes the distance between the subject and the antenna
> and keeps RF off of the feedline would be better than one that
> does not. In other words, I would try to
Agreed.
> stick with well matched resonant dipoles with good baluns and
> coax, and operate on the lower HF bands. And, of course, many
> hams have lots of fun running QRP.
>
Maybe I'm missing something in regards to the caveat: "and operate
on the lower HF bands".
I am of the understanding that a 1/2 wave or 1/4 wave 'ground plane'
(the ground radials *do* form a choke of sorts to RF currents besides
acting as the counterpoise in 1/4 wave ground plane) antenna would
also fit into the prior category of antennas "that maximizes the
distance between the subject and the antenna and keeps RF off of
the feedline", therefore, 10 Meters, 12 Meters (and possibly 17 and
20 Meters depending on antenna design) are a possibility for operating
on a small lot with an eye towards keeping RF exposure levels low.
Full disclosure: I have a 1/2 wave "ground plane" antenna (10 M
Ringo look-a-like with 'aftermarket' radials attached) with the base
20' up in the backyard near the back lot line and don't experience
RFI on any electronics devices when running 100 Watts (that is
usually the first indicator that there may be unusual RF levels
present.)
I haven't any experience with horizontal beams and RFI on 10 - 20 M
so I can't speak on what RF exposure/RF effects are beneath those
types of antennas sitting atop 50' or 75' masts next to a house.
On the other hand, I trip my main GFI breaker instantly when
transmitting on 80 M with a dipole placed just over the house at
the same power level.
Which brings to minmd this thought: Any lower freq/longer wave-
length HF band antennas comprised of simple balanced dipole
radiating elements are going to create 'fields' (literally: magnetic
and electric energy-containing fields which store energy from RF
cycle to cycle in the proximity of that dipole on the order of 1/2
wavelength or so from that antenna - 127' for 80 Meters for
instance) so upon further review I'm not sure that a caveat about
"well matched resonant dipoles with good baluns" is as much a
part of the solution as establishing *pure* distance from the
radiating elements to sensitive human or equipment; on a small
city lot that is not an easy thing to do!
Maybe an inverted V for the lowr HF freqs hoisted in the middle
using a 75' tower as a support that also supports a multi-band
horizontal beam for 10 - 20 (or so) would provide sufficiently
low RF levels for 100W on the lower freqs and support 500W
or so on the higher freqs (for those bands on the Horz beam.)
Jim P // WB5WPA //
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|