[Milsurplus] BC-230 questions and observation

Sat, 15 Mar 2003 17:50:48 -0500

Hi,

More or less - a tapped tuned tank driven off of a vacuum tube plate will
look like a current source provided the Q of the coil does not get into the
act. In the case of a reasonably designed transmitter this will be the case.

If you measure the harmonic levels out of a transmitter with a constant
impedance instrument you will get the first piece of data you need.

The next step is to translate it assuming a constant current drive. Since
P=I^2 * R and I^2 is constant the power will go up linearly with the
resistance. In other words 0.1 watt into 50 ohms would scale to 1.0 watts
into 500 ohms.  To translate in db in this case you would use 10 log R.

The higher in frequency you go on an antenna the higher it's radiation
resistance gets. If you drove it off of an ideal current source you would
get more and more power delivered into the antenna as the frequency went up.
The amount of energy would also vary but that's a different thing.

The other thing to consider is that the directivity of an antenna changes
quite a bit as frequency goes up. Short antennas tend to shoot a lot of
power straight overhead. As frequency goes up the power begins to shoot
towards the horizon. If frequency gets high enough the pattern starts to
break up and multiple "lobes" form. None of this is easy to predict without
specific data.

All of the manufacturer data is generally done on properly installed
equipment running with the right supply voltage and correct tuning. If any
of those three are not what they should be then more harmonics are the
likely result. A poor ground on the installation is a way to get more
harmonics. A radio tuned to put out more power than it is rated for is an
*excellent* way to get harmonics on this vintage of radio. Any tube radio
run with to low a voltage on the plate will be prone to harmonics unless it
is retuned for the lower voltage and output power is reduced correctly.

With the exception of minor information on tuning *none* if the information
above would be available to your average pilot in the pre-WW2 era. It wasn't
part of the course of instruction. It still isn't for that matter.

    Take Care!

        Bob Camp
        KB8TQ

----- Original Message -----
From: "Hue Miller" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 15, 2003 4:32 PM
Subject: Re: [Milsurplus] BC-230 questions and observation

> Thinking about the harmonics from the BC-230 issue some
> more:
>
> Jack, i think your test would be valid, if you use a purely
> resistive load. I also think loading the BC-230 with a pure
> resistance, like first 35 ohms to represent a vertical 1/4
> wave, and next,  6000? ohms, to represent a vertical 1/2
> wave, will give you a figure for harmonics that will be real-life
> and worst case, both, because the harmonics see an antenna
> that is also not reactive either capacitive or inductive. That
> simplifies things. I also think this is a valid worst case, because
> the aircraft horizontal antenna, which is a "low, horizontal"
> antenna, has much lower radiation resistance than the vertical.
> The Sandretto book points out that with this coiltap system,
> it is less efficient for low radiation resistance values, because
> the low radiation resistance tends to be swamped by the
> actual resistance and losses of the tank coil. In fact, for
> one example on ~3000 kcs, they calculate a matching circuit
> efficiency of about 50%, and this even reckoning the antenna
> coil Q is a generous 300.
> I think the actual very worst case, for harmonic radiation, is
> the vertical, hi-impedance 1/2 wave. I don't know any easy
> way to do this one, with your spectrum analyzer, with its 50
> ohms standard.
> Hue Miller
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