[TowerTalk] Grounding, portable generators, field day

Fri Jan 21 10:56:43 EST 2005

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.

73
Gary K4FMX

K8RI on Tower Talk wrote:
> 
> ----- 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
> 
>>
>> _______________________________________________
>>
>> 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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>>
> 
> 
> _______________________________________________
> 
> 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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