[Lowfer] HIFER POWER?

Sun, 07 Mar 2004 05:46:10 -0600

Hi Ed,

As you mention, the field strength limit in Part 15.225 for 13.553 to
13.567 MHz is 15,848 microvolts per meter at 30 meters, which is an EIRP
of 7.53 mW.  For a perfect dipole in free space or quarter wave vertical
over perfect ground, the limit will be about 4.59 mW to the antenna
feedpoint, assuming a gain of 2.15 dB for a perfect dipole over
isotropic.  For real antennas, the actual pattern and gain need to be
accounted for and the power at the antenna reduced, or perhaps
increased, accordingly. Flat feedline loss and any loss due to mismatch
or matching networks between the feedline and antenna will allow an
increase in transmitter power.

A 50 foot vertical would have a little positive gain which you would
have to estimate or model in some way.  I did a quick EZNEC model that
shows a gain of 2.3 dBi at an elevation of 49 degrees, less than I
expected, but I made up my own antenna details. At reasonable lower
takeoff angles, the antenna is down at least 2 db from a quarter wave
vertical because it will be a little too long for the frequency.  My
guess is you would be better served with a dedicated 17 foot vertical,
but the 50 foot antenna would work pretty well for a trial.

The full range of frequencies from 13.553 to 13.567 isn't really
available as the allowed field strength drops to 334 microvolts/meter
immediately outside this range, a reduction of over 33 dB.  Over 14.010
MHz, the rules require emissions to be down over 43 dB from the 13.553
to 13.567 limit.  Since this is too close to the beacon transmit
frequency for a low pass filter to be effective, the transmitter needs
to be fairly clean, but you'll still likely need a LPF to reduce
harmonics.  My measurements on the Epson oscillator currently used for
the RT beacon show emissions outside the 13.553 to 13.567 range are down
over 50 dB, so these requirements are not difficult to meet.

Good luck and I'll look for your beacon when you get it running.

Roger
AD5T

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Ed Phillips
Sent: Saturday, March 06, 2004 7:08 PM
To: [email protected]
Subject: [Lowfer] HIFER POWER?

I have just looked at part 15.225 and find a field strength limit, with
no mention of antenna configuration.  If my calculations are correct the
power limit would be about 7.4 mW into a short vertical over ground. 
Approximately right?

	Big question is about the antenna.  If I can use my present 50
foot
vertical it will be very simple for me to get a HIFER beacon on the
air.  Am I overlooking something important?  [I realize that this
antenna may have a bit of gain, but can adjust the power for that.)

Ed
_______________________________________________
From the Lowfer mailing list
Send messages to: [email protected]
To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer