On Mon, 2010-03-01 at 14:38 -0600, Stuart Rohre wrote:
> Good question John. Modern rigs have to meet a tighter level of
> suppression of off frequency signal in transmit than the rigs of the day
> when Low Pass filters were in demand to help with the TVI problems. At
> one time, old TV sets had an IF right in the 15 meter band, to really
> make things hard!
Yup the 21 MHz IFs did cause more than a little neighborhood unrest.
>
> Also, in those days of the 50's, more rigs were home built and might
> have minimal TVI suppression. Today, most TV is digital, and on
> frequencies up in UHF well removed from harmonics, which aren't there
> since the transmitters are better.
That's not necessarily true. Temporarily most VHF stations went UHF for
digital and many went back to their VHF channels, more high VHF than low
but there are a few stations still on the low VHF channels and running
digital. Its harder to get the antenna bandwidth needed for good
reception but a few have done it.
Thing is channel number may or may not be the old analogue channel RF
frequency and the other thing is any interference just results in no
picture or sound, without incriminating sounds of SSB or voice or lines
in the picture.
> Thus, I too have questioned leaving
> a LPF in line if I am trying to get maximum clean output.
>
> I would say the definitive answer might be to measure the effect of the
> LPF on any harmonics you can find in the rig output, and measure the
> loss of the signal going in to the signal going out of the filter, (the
> desired frequency).
Its vital that the low pass filter be AT the output connector of the
rig, not a few feet of coax away. Because that few feet of coax can act
as an impedance transformer matching the typical filters series resonant
shut trap at the connector ends of the filter to a high impedance at the
transmitter and actually increase the harmonics to the antenna at the
trap frequency.
>
> Also, be cautious about flea market/ swap meet filters, even commercial
> ones, if they had easy to access adjustments. They may have drifted off
> with age, or worse, someone may have twisted the screwdriver adjustments
> "just to see what happens". Sweep frequency testing is the best way to
> line up a filter. Most labor intensive is to do a point by point
> frequency chart with a graph made at the end to show the filter
> response. Of course, both kinds of tests should be done into a well
> shielded dummy load.
Definitely filters are friable, rated at 200 watts on a KW amp, can mean
broken parts or worse open circuits causing high SWR to the new rig. And
sliding contacts in unmoved variable capacitors often have gone open
after decades of sitting in a damp barn or basement.
>
> -Stuart Rohre
> K5KVH
>
73, Jerry, K0CQ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|