Topband: Re: 160M Bandpass filter

[Larry Molitor]

Since KMIK (as in MIcKey mouse/Disney), 1580 KHz, puts +5DBm out of the 
coax on my topband TX antenna, I feel that my experience in this area might 
be worth a little bit to someone.

Notch filters of the common configuration have shown me to provide, at 
best, 23 DB of 1580 rejection and when used with an impedance bridge, 
"pull" the impedance enough to make the measurement worthless. Notch 
filters are the easiest to build and you pretty much get what you pay for. 
Minor cases of BCI, especially from a single station on the lower end of 
the band can be solved with this approach.

Just today, I designed, built, and tested a 7 pole, .5DB ripple, Chebychev 
high pass. It provides 18.5 DB rejection at 1580 and around 60 DB at the 
bottom end of the BC band (limited by layout). High pass filters are a 
little more complex than notch filters, but are probably the best bet for 
BCI problems stemming from a bunch of stations from the middle to low ends 
of the BC band. This filter works well with my Flag RX antenna only because 
the BC (and topband) signals are 35 DB or so below the levels seen on my TX 
antenna and that's low enough for MIcKey not to be a problem.

With a fairly simple two section bandpass, I see 30 DB rejection at 1580 
and in excess of 60 db at the low end of the band. I have used this filter 
for the last ten years or so with outstanding results. A well designed 
bandpass filter, in my experience, is the best bet, however....

And this is a big however....  It is almost impossible to properly tune 
even a simple bandpass filter  without a network analyzer or an equivalent 
pile of test equipment. I have tuned them at home (before I got my own 
network analyzer) by using a return loss bridge setup, but when I would 
look at them later with an analyzer, the bandpass was pretty ugly. Over the 
years, I have built hundreds of these things from VLF through 500 MHZ and 
have never once seen one that did not need to be tuned!

Elliptic function high pass filters are favorites of some folks, but I have 
never had much luck with them. They are relatively hard to design and build 
properly and I'm not smart enough to ever get the notches where I need 
them. If you can get one of these to work for you, it's an excellent solution.

If Multi-Multi operation, and not BCI, is your problem, a good bandpass 
design is probably the only option for you.

In the last two weeks, I've spent around 80 hours working with filters and 
such trying to find a more elegant solution to the problem of measuring the 
TX antenna impedance in the presence of MIcKey on 1580. I can do it by 
brute force methods, but so far anything else I've tried has been a waste 
of time. Another project on the bench, which uses filter technology, is a 
RX antenna switch and control box. Hence the 7 pole highpass filter work 
today. I would very much like to correspond with anyone, directly, who 
might like to share related experiences and technology. An occasional 
reality check is a good idea, for me at least!

73,

Larry - W7IUV


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