[Milsurplus] BC-230 harmonics

[Hue Miller]

----- Original Message ----- 
From: "Jack Antonio" <scr-287@sbcglobal.net>

> Some questions
> 
> 1. Do these numbers seem reasonable, or is my test setup needing some fine tuning?
> I do plan on making a resistive divider to feed the analyzer,
> making sure the analyzer  sees a 50 ohm source.

-I haven't used a spectrum analyzer, but i am thinking as long
as the spectrum analyzer is isolated so it doesn't affect the 
load impedance, and its gain is good enough, it doesn't need
to see a 50 ohm source, because all you're interested in
his harmonics down in db from the standard (fundamental),
not actual levels, like dbm.

> 2. Even if these numbers are meaningful, would they be applicable to a 
transmitter running a short wire antenna(instead of a stepup transformer
> into a 50 ohm load).

-That's a good question too. In thinking about how that loading
circuit works for short capacitive antennas, i came up with this:
The short antenna is seen as a low resistance, the actual radiation
resistance, in series with the antenna capacity. This is the standard
model of the short mobile antenna. The coil of the transmitter is
tapped to an inductance point that resonates with this capacity,
typically 50-100 pF range. Then the Rr resistance, less than 10 ohms
typically, is in series with this series tank circuit, where max. current
is flowing. Not a perfect match, not as good i think as having a 
separate loading coil to cancel the Ca and THEN find a tap on
the tank coil, but it works. So, i am thinking, if you try to match
this kind of antenna to the transmitter, the coil tap setting will be
way up the coil compared to the lower tap setting for matching
a pure resistance of low value, say 10-50 ohms.
I am not sure this test is valid for harmonics anyway, because
consider this: at different harmonics, the harmonic can see a way
different antenna impedance. Maybe testing of radiated harmonics
is the only way to go. Or, to keep with purely resistive loads.
I think the "worst case" harmonic output would be with a max.
impedance antenna, say a 1/2 wave wire, or about 4000 ohms
pure resistive. This would be a tap way high on the coil, each
harmonic would be a multiple of 1/2 wave so the antenna would
present a high impedance match for the harmonic too- in other
words both fundamental and harmonic would be well matched
to the antenna. Also, i am thinking the closer the tap is to the 
plate tap, the more harmonics are favored, because the lower
part of the coil from the tap to the ground, acts more like an
RFC to keep harmonics above ground.
Hue