From: Jon Ogden <jono@enteract.com>
> WOW! You are asking some tough questions here!
>
> First of all, indicated forward power is likely to be the SUM of the
> forward and reflected power. One way to check is to tune the amp into
> your antenna. Then with the same level of drive, tune it into a 50 Ohm
> dummy load. It should come quite close to verifying that.
Ok, 1.8 kW indicated into 2.3:1 vswr impedance load, with
300 or so reflected was previous data.
Now, because of some strange behavior by the P-3000 gadget,
have decided to switch to Bird instruments: Model 43 watt meter,
with 100, 500, and 2500 watt slugs used; and Bird Coaxial Load
Resistor Model 8890-300 dummy load. (Suspicious of the diodes
in the P-3000 coupler, vswr reading increases with increasing input
power to the load, and unit did not read 1:1 into the Bird load except
at very low power, then increased with power into the load, so am
switching to the Bird 43 wattmeter).
1. With same drive power as before, about 100 watts (as
measured by the P-3000 before replacing it with Bird) read following
data into the 2.3:1 antenna set up at 28055 kHz:
Indicated forward power = 1500 watts
Indicated reflected power = about 250 watts
2. Increased drive to yield 1820 watts forward (as close as I could read
on the Bird) using the 2500 watt slug. Rotating slug, found about
310 watts reflected, or very similar to earlier results with the P-3000,
except for the apparent differing drive requirement.
3. Connected the exact same transmission line terminal (as used
from the rig to the antenna) to the same type of transmission
coax to the Bird dummy load.
Applied exact same drive power as used in (2) above; Bird read
2.1 kW forward, and absolutely no reflected power with the slug
reversed !? So, find I get more output power into nearly perfect
50 ohm system load for same input drive power.
4. Reduced drive power so that exactly 1500 watts forward is
indicated on the Bird into the dummy load. Replaced Bird
2500 watt slug with a 100 watt slug. Read absolutely no reflected
power. Turned off linear and read drive power to be almost
exactly 45 watts when driving 1500 watts into the Bird dummy load.
5. Leaving drive power sitting at 45 watts when ON, returned to the
2.3:1 antenna set up. Using the Bird 2500 watt slug again, found
that the 45 watt drive to the linear was sending 1250 watts indicated
forward on the transmission line, and using the 500 watt slug,
about 200 watts reflected.
6. Increased drive power so that 2500 watt slug was reading 1820
watts, as near as I could read the Bird with a magnifying glass,
and found that just about 300 watts was being reflected, again
using the 500 watt Bird slug. Required drive, to reach these
numbers into the 2.3:1 antenna system load, was about 140
watts, read with the 500 watt slug.
So net output from the antenna is about 1500 watts under the
conditions here in no. 6.
Conclusion: More drive power is required to yield a given
power into a load which differs from 50 ohms, in fact it
seems to require the drive power that would send, into
a good 50 ohm load, the sum power of the indicated
forward and reflected. 140 watts drive into the linear
would produce well over 2 kW into the dummy load,
see point 3, above.
So, the operating conditions as described above
in point 6 result in legal output of 1500 watts when using my
2.3:1 beam on 28055 CW. My P-3000 is sick. I no longer
need to use an antenna tuner, as it appears that most,
if not all of the reflected power from the line/antenna terminal
mismatch, is re-reflected back up the line, to once again
have about 85% of it radiated, and 15% re-reflected once
again, and so on. The linear is outputting sufficient power
to cause the antenna to radiate 1500 watts, and is
in fact itself only amplifying the drive power to 1500
watts as in point 6. The reflected power is not seen
by the tube within the amp, and no harm is done to
the linear.
OR: maybe the linear is, in fact putting out over 2.1kW,
with the 140 watts drive needed while only some
1820 or so is indicated forward with some 300 reflected, and
the balance dissipated with in the amps "virtual" source
resistance, with the antenna radiating only 1500 watts, as
Jon opined. Then maybe I should continue to use the
tuner?
73, Jim, KH7M
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|