[Lowfer] HIFER POWER?

[Ed Phillips]

Roger Thompson wrote:
> 
> 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

Roger:

	Thanks for the note and seems to confirm what I thought.  I'm
particularly interested in your EZNEC simulation, as the high-angle
prediction agrees almost exactly with what I get here using MacMININEC,
a Mac-based program I gut from Chris Smolinsky (sp?) a long time ago.  I
modelled the tower as a bottom section 25' long and 8" diameter with a
top section 25' long and 2" in diameter, which I think is a reasonable
approximation.  Usable, but not the greatest.  Here's the part I'd like
a comment on:

	In playing around I tried ungrounding the tower and instead using an ~
20' length of wire grounded at the base of the tower and leaning away
from the tower at the top.  Actual metric parameters:

TOWER (NEITHER END CONNECTED):
1. x=0, y=0, z=0 at bottom, x=0, y=0, z=7.5 at top, 0.2 meter dia
2. x=0, y=0, z=7.5 at bottom, x=0, y=0, z=15 at top, 0.025 meter dia

RADIATOR (FED AT GROUND END):

	The impedance comes out to about 12.8 + j1.7 ohms. The vertical pattern
shows a maximum at the horizon, down to about 0.8 power at 15� and 0.58
power at 30�.  Just over a dB horizontal F/B directed, (IF I understand
the geometric conventions), AWAY from the tower (as if it is acting as a
reflector).  "Gain" (whatever that means) is 7.9 dB.  

	Does any of that sound right to you?  It would sure be simple to
implement.

Thanks,

Ed
x=0.2, y=0, z=0 at bottom, x=0, y=2, z=5.9 at top, 0.005 meter dia