[Towertalk] Commercial 300W Amp

[Pete Smith]

At 09:56 PM 10/15/02 -0500, Phil - KB9CRY wrote:
>Neighbors don't want to hear about, don't know anything about, PRB-1. What 
>I used to say, when I lived in a close in neighborhood, is that my station 
>is licensed by the FCC.  I transmit "radio signals".  Your TV or telephone 
>is supposed to pick up TV or telephone signals.  If is it picking up my 
>radio signals, then there is a problem with your equipment not mine.  Then 
>you whip out the filters and such, if they are willing, and show them the 
>solution.  There is an engineering solution to every rfi problem.  As long 
>as you are clean and properly grounded and filtered (low pass filter on 
>the output), then the problem is always (most always) at the other 
>end.  Now you have to use diplomacy techniques.  (Not printed in the ARRL 
>manual!)  Good luck.

Another point worth mentioning -- all of these issues (antenna changes, amp 
or no, RFI) are tied together by a common unit of measure, the decibel 
(dB).  For example, if you go from 100 watts to 1 KW, assuming both signals 
are equally clean (within FCC specs), then your signal will go up by 10 dB, 
not only to the stations you want to work, but to devices that might 
receive interference as well.  Your 300-watt amp would amount to adding 
~4.2 dB.

Now let's assume that before you added the amp, you were right on the brink 
of causing interference to any or all of those devices.  Then there will be 
10 (or 4.2) dB worth of interference to get rid of -- not that much, once 
you figure out how the RF is getting into the device.  In terms of what's 
needed, the cure for 4.2 dB of interference and 10 dB worth are essentially 
the same.  The RFI reflector (rfi@contesting.com) is indeed a good place 
for advice on this, but the ARRL RFI manual is a good resource to start 
getting smart with.

Turning back to antennas, typically people don't experience as much 
difficulty with RFI as a result of improving their antennas.  For one 
thing, it is hard to pick up 10 dB through changing outdoor antennas.  A 
typical small tribander like the C-3 has only about 5.5 dB gain over a 
dipole installed at the same height.  The Hexbeam has less.  For another, 
the gain is only (hopefully) in one direction, while to the back and sides 
your signal may actually be weaker than before.  Finally, people typically 
put their improved antenna higher or in a different location, which may 
actually decrease the amount of signal being delivered to devices nearby 
that are susceptible to interference.

The other figure of merit (besides the dB) I like is the $.  Simply put, 
you need to look at the dB/$ you get from any improvement to your station, 
then factor in extrinsic factors such as RFI, XYL relations, etc., to 
arrive at a rational improvement path for your station.  If you're 
currently running barefoot, then the cheapest dB of improvement you can 
find would be to add a used linear, like an SB-220, Drake L-4B, etc. at 
~$500-550.  Those dB only cost $50-55 each.

Once you've added the amp, though, most of your dB will come from changes 
in your antenna system, feedline, etc.  Jon's suggestion of LMR coax is a 
good one.  I would consider almost any good line but Belden 9913 (which is 
very prone to water infiltration).  Below 30 MHz, losses aren't huge, even 
with 100 ft of old-fashioned RG-213 (about 1.3 dB at 30 MHz, less at lower 
frequencies).  Here again, if you apply the dB/$ metric, you can look at 
the difference in price between RG-213 and LMR-400, for example, and figure 
out how much THAT dB (the difference between .3 and 1.3 dB loss) will cost.

Generally speaking, hams seem to disdain doing a lot of pre-installation 
analysis.  "Put 'er up and see how she plays" is still alive and well.  On 
the other hand, with local building permits getting more expensive and 
harder to get, plus the impact on available time and money, people may be 
starting to think more first.  It sounds like that's what you're doing, and 
it is a very good thing!


73, Pete N4ZR
Sometimes a tower is just a tower