[TowerTalk] Grounding, portable generators, field day
K8RI on Tower Talk
k8ri-tower at charter.net
Thu Jan 20 23:15:39 EST 2005
----- Original Message -----
From: "Jim Lux" <jimlux at earthlink.net>
To: "K8RI on Tower Talk" <k8ri-tower at charter.net>; <keith at dutson.net>;
"'towertalk'" <towertalk at contesting.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 safety
>>limits.
>
> 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 problem.
>
> 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 calculation.
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.
>
> 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 safety
> rules.
>
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.
>
> 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)
www.rogerhalstead.com
>
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