[HCARC] Field Day Antennas

[Kerry Sandstrom]

Well Gary,

If we were running Class A or B then setting up temporary antennas in a 
temporary location is part of the purpose of Field Day, but when you are 
Class F I don't think that is true.  Class F is a more or less permanent 
station co-located with an emergency facility of some sort such as the 
Red Cross.

Technically, operating several one hundred Watt transceivers in close 
proximity is a challenging problem.  Transmitters don't just transmit on 
a single frequency but on a main frequency plus numerous other 
frequencies depending on their spurious emissions.  These include 
harmonics, images and broadband noise from the synthesizers.  Receivers 
are equally unpleasant responding not only to the frequency it is tuned 
to but also to images and various spurious frequencies.  in addition the 
synthesizers in receivers also have broadband noise that mixes with 
strong nearby signals. Finally, when strong signals are present the 
various non-linearities in the receiver will generate even more spurious 
signals.  A typical dynamic range spec for receivers is around 100 dB.  
The minimum signal power a receiver can detect is around -120 dBm.  The 
maximum signal levels the receiver can see before dynamic range starts 
becoming a problem is -120 dBm + the 100 dB dynamic range = -20 dBm or 
10 microWatts.  How far apart do you think a receiving antenna needs to 
be from a transmitting antenna before the 100 W output is below 10 micro 
Watts at the receiver?  How far apart would they have to be before you 
wouldn't be bothered by a 50 dB down second harmonic from the 100 W 
transmitter.  The big operations often use special RF filters on both 
transmit and receive to reduce these problems.  Note that the difference 
between the transmitted signal and the received signal is +50 dBm (100 
W) to -120 dBm or 170 dB. That is a lot of dB!

 From a practical point of view, I think trying to run 3 HF rigs from an 
area as small as the roof of the Red Cross building and the space 
immediately behind the building will be quite challenging.  Gary, as a 
Navy guy, I'm sure you've heard about the 'rusty bolt' problems on ships 
in which poor or corroded metal to metal joints, intentional or 
otherwise, generate noise and spurious signals due to the non-linear 
properties of the joint.  How many 'rusty bolts' do you think are in the 
vicinity of the Red Cross building?

I think we would be fortunate if we can actually run a station on 80/40, 
a station on 20/15/10 and a GOTA station on the courthouse lawn.  I 
recommend that as our goal and see if we can actually do it.

By the way, I won't be able to take part, however that doesn't change my 
recommendation.

Kerry