Navtex-Fan wrote:
Z = 241-j1812 Gain = -28.6 dB SWR = 278
I'm assuming that the mismatch is "solved" by the use of the 4:1 UnUn..

Nope, not necessarily. Most 4:1 designs assume a resistive
load and do not handle reactance very well. A 4:1 device
might make the problem worse.

If I model a base loading coil of +/- 550 microH , the following results:
Z = 165-j14 Gain = -26.9 dB SWR = 3.3

As long as your receiver AGC has enough gain to overcome
the loss in the antenna, why worry? Internal receiver noise
is usually negligible compared to atmospheric noise at
518 KHz.
--
73, Cecil http://www.w5dxp.com

On 2007-12-28, Cecil Moore (75.23.42.181) wrote in
message


Navtex-Fan wrote:
Z = 241-j1812 Gain = -28.6 dB SWR = 278
I'm assuming that the mismatch is "solved" by the use of the 4:1 UnUn..

Nope, not necessarily. Most 4:1 designs assume a resistive
load and do not handle reactance very well. A 4:1 device
might make the problem worse.


Thanks for the reply Cecil,

Are you suggesting I should not use a unun at all?
I have also experimented with the commercial "magnetic balun" which has
a 9:1 ratio. The only difference I could tell was a _slightly_ lower
noise level.


Cheers - Dirk

Navtex-Fan wrote:
Are you suggesting I should not use a unun at all?

Unbalanced antenna to unbalanced receiver with a
very large dynamic range may result in diminished
returns for your effort. But if you want to use it
as a learning experience, it might be worth it.
--
73, Cecil http://www.w5dxp.com