Good afternoon friends,
following the comments and suggestions I got on the negative performance of
my rotable Flag, I checked everything and discovered that the big noise was
introduced by the preamplifier, which has been working very well for years
with any kind of antenna.
Gary, KD9SV, wrote me:
-----------------------------------
Since building the unit you have I have started putting a 24pf capacitor
across the 100k resistor to prevent the pre-amp from oscillating when source
and load
impedances are not close to 50 ohms. What kind of matching transformer are
you using on the Flag antenna?
------------------------------------
The matching transformer is a 18:1 binocular BN73-202 (3 and 12 turns).
The impedances seen by the premplifier are:
- Flag R 57 +jX 0 = SWR: 1.15
- Pennant R 43 -jX 5 = SWR: 1.22
- Pennant with outdoor preampl. R 60 -jX 78 = SWR: 3.9
- 4 square mini-array R 70 -jX 6 = SWR: 1.45
Despite the best matching, ONLY with the Flag the preamplifier was
oscillating!
I substituted it with a spare ICE mod.123B and....beatiful, no noise with
it.
But I could not survive without the KD9SV Dxpedition model as it containes
also the indispensable Front End Saver ( see
http://johnjeanantiqueradio.com/gary.htm or
http://www.radiobooks.com/products/svdx.htm ) so I took the courage to put
the iron gun inside it and add that small capacitor as suggested by Gary.
Really an easy task even for a non technician like me, and ...WHOW
everything was OK!
Now the Flag is quiet as the Pennants, with the output level abt one s-unit
higher and the expected F/B from 20 to 30dB.
Its building is more demanding than the Pennant but, beeing rotable, has the
great advantage to cover all the directions.
And now I'm going ahead with the K9AY loop building and tests......
Many thanks to all who wrote me directly with comments and CU on the Band!
Best 73
Luis IV3PRK
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