[R-390] Matching network

[Bob Camp]

Hi

If the purpose is general listening then a "matched" antenna may not be 
the best bet unless you have room to put up lots of them. If you are 
like most people you have limited room and the real question is how to 
get the most out of the antenna you have.

More or less a dipole is at it's minimum impedance at resonance. The 
magnitude of the impedance goes up as you go away from resonance.

If your objective is to get as much out of the antenna over as wide a 
band as possible then you want a higher impedance on the antenna. 
Simply put 120 ohms will work better than 75 and 75 will work better 
than 50 ohms. This holds true up to about 200 ohms or so for normal 
dipoles. After that point the bandwidth does not get any better.

This is not to say that you will get more signal out of the dipole at 
resonance when you load it in 120 ohms. If fact at resonance you will 
get a bit less. The point is that by say 20% off resonant frequency you 
will be even with the matched load and past that you will beat it.

The total difference is the square root of the ratio of the impedances 
so for a six db change in signal you would go from 50 ohms to 200 ohms. 
Even with a fairly compact antenna you should have enough to get a 
R-390 going below 10 MHz in the evening with either feed impedance.

	Take Care!

		Bob Camp
		KB8TQ

On Aug 31, 2004, at 4:26 PM, Charles B wrote:

>
> Hi all,
>
> what can I do to get a better match of balanced antenna (dipole) to my 
> R390A.  At the moment the antenna is attached to J106 and in the same 
> configuration as shown on http://www.r3980a.com/html/feedpoint.html  
> top picture.
>
> Would an antenna tuner help such as the MFJ 959B?
>
> Chuck
>
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