With a beam why do you add the gain in when you are below it? You are in
more of a null there than you would be with a dipole.|
K8RI on Tower Talk wrote:
----- Original Message ----- From: "Jim Lux" <email@example.com>
To: "K8RI on Tower Talk" <firstname.lastname@example.org>; <email@example.com>;
Sent: Thursday, January 20, 2005 9:09 PM
Subject: Re: [TowerTalk] Grounding, portable generators, field day
At 04:13 PM 1/20/2005, K8RI on Tower Talk wrote:<snip>
When you can safely get within a few feet of a 75 or 40 meter dipole
running a KW safely and you might be running a tribander at 30 feet
with a 100 watts it's going to be pretty difficult to exceed any RF
I think the RF safety issue might be more of a problem with VHF and
UHF rigs. There's that picture in QST last year with the whip antenna
on the picnic table next to the operator, for instance.
Agreed, but even with whips and 50 watts you can get very close.
There have also been people who run 4-squares at field day (that one
had a picture on their website with plastic net fencing around the
antennas for RF safety).
Since the typical field day station will have more than 1 transmitter
operating simultaneously. I don't think the typical field day setup is
going to fall into what the FCC calls "categorically exempt from
routine station evaluation". The "safe harbor" values in OET65B page 3
probably assume single transmitter at a time. If you're running 100W
on 10 or 15, you'd also have to do an evaluation.
Remember the calculation is done for *average* power, not peak and takes
into account duty cycle.
The DUTY FACTOR for CW is 40%, SSB phone 20%, FM, RTTY, and AM 100%, but
The evaluation can be done well before setup. You can work the
calculations backwards but substituting in values. If you know you are
running 100 watts out (or any particular power) through 100 feet of
coax, you can figure in the gain of the antenna and the height and come
up with the safe distance. I would think it would be the uncontrolled
environment which is based on the average over 6 minutes. For contest
operation this is going to be around 50% unless some one calls CQ for 6
minutes at a time. There are those who do.<:-)) But for safety's sake
(your's doing the calculations) figure 100%. It isn't going to make a
big difference at FD power levels.
Use the power calculator at http://n5xu.ae.utexas.edu/rfsafety/ or
download the program.
I'm using the following as expamples. Every one should do their own
calculations. BTW, you can subtract line loss as the calculations are
based on power into the antenna or array.
Let's say I'm running 100 watts at 100% to a 4 element beam which gives
8dbd gain. on 10 meters. That's 10.2 dbi and calculates out to 33.93
feet for the uncontrolled environment. But it's SSB which is only 20%
and figuring the operator is really long winded give them 100% instead
of 50% for the 6 minute average. That comes out to 15.2 feet for the
uncontrolled environment. Put the beam at 22 feet and the public could
stand under it.
With CW at 40% it's 21.48 feet so you'd need the beam to be a tad less
than 30 feet for the general public. For hams? It'd only be 9.63 feet.
Two meters is really the worst case scenereo. On 147 MHz with a 100
watt CW signal and 12 dbi gain the distances are 11.84 and 26.41 feet.
OK so what about the whip in the center of the table. Give it 3 dbi for
a 5/8 whip and 50 watts continuous the calcs are 4.73 and 10.51 feet.
Certainly not a good thing. Drop it to 50% duty cycle and it becomes
3.36 and 7.45 feet. You might want to put the whip on a post to be legal.
What about 75 meters? how about 1.58 and 3.48 feet respectively.
Unless you drape the thing over someones shoulders it shouldn't be a
Another example is whether contesting operation can legitimately apply
the 20% duty factor for "Conversational SSB" in Table 2, Page 14.
(Especially if you fall asleep with your finger on the parrot <grin>)
For most contesters you bet. You figure the additional factor based on
the 6 minute average. So if they are realy long winded give then 100%
for that, which still leaves them with the 20% for SSB. Even a good
loud snore would qualify for the 20%. OTOH a steady tone would not.
Typically a good contester is not going to be transmitting more than
receiving. That would be a minimum of 50%, so you have 20% for SSB and
50% for duty cycle which results in a factor of only 10% for the RF
However it should become evident that whether they are given 50% or 100
% for duty cycle it will make little difference in the distances.
Another antenna that might need some analysis (since it's not in the
OET65B list) would be an inverted V for 80/75. At FD, you might have
the vertex/feed up at 40 feet but the ends of the dipole (where the E
field is highest) might only be 10 feet off the ground.
See the above figures. You can get close enough to spit on the thing
and still be safe.
So... the analysis for any of these antennas might be trivial, but you
still have to do it, and you have to have the "artifact" to show that you
Just do the calculations before ever setting up. You have the sheets
and as long as the distances are greater than the minimums you are OK.
If the antennas are far enough apart they don't bother each other they
should also be safe even combined.
did. It might just be a worksheet in the back of OET65B or piece of
paper with the (trivial) analysis on it (worst casing from the tables
in OET65B, for instance), documenting the assumptions on duty factor
and power. If you spend more than 15 minutes on it, it's probably
overkill.It should be something on every ones minds, but I really think that the
RF safety is probably so over blown (no one knows for sure what levels
are safe) that even the whip in the center of the table is plenty safe.
Actually, though, I think electrical safety is more of an issue at
field day than RF safety. The comment about RF safety was more that
just because you're at field day doesn't relieve you of all the usual
By the way, as far as the assumption of field day running 100W goes
(or even 5W), it's rumored that some big FD ops use a "specially
calibrated" power meter.
I set mine to impress. People wonder how I can get so much power out of
a couple of transistors and a 12 volt supply drawing 20 amps which the
756 Pro does at 100 out on CW.
Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
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.
TowerTalk mailing list
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.
TowerTalk mailing list
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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