Along with what Jerry, W4UK, wrote a couple sets of homemade W3NQN filters
and homemade stubs used for SO2R, 1500 W, RTTY contesting with a single
tower for supporting all the antennas. Yes, you can only get within ~10
KHz of the second harmonic frequency but that still leaves a lot of the
band available.
W3NQN filters (homemade) are used. The initial set of the filters were
tuned using just a MFJ259 antenna analyzer. This was over 10 years ago.
Since then I've built a VNA. Sure, the MFJ259 tuned filters were not as
sharply tuned as could be accomplished with the VNA, but they still
functioned well in the high power RTTY SO2R environment.
The cost per filter can be quite inexpensive if you make them yourself and
limit the throughput power to 100 W max. Turns out the most expensive
component(s) for each filter are the SO239 connectors. You can even
eliminate a couple of the toroids per filter for the 10 - 20 m filters by
using air coils for the input and output parallel circuits. The
enclosures are made from aluminum flashing.
So don't overlook fabricating your filters as the experience and cost
savings are very worthwhile.
73,
Mike, K4GMH
On Sun, Mar 20, 2016 at 11:57 AM, Jerry Flanders <jeflanders@comcast.net>
wrote:
> When I operated SO2R years ago I had bandswitching filters ahead of the
> amps and homemade stubs after. I could operate anywhere except within 15-20
> Khz of the second harmonics.
>
> 50 ohm coax is not required for the stubs - I was using free 75.
>
> Jerry W4UK
>
>
> At 12:21 AM 3/20/2016, Jim W7RY wrote:
>
>> George Cutsogeorge, W2VJN's, book is really great for using tuned stubs
>> (coax cable) for low notching out other bands for SO2R. I use on on my 80
>> meter antenna to keep from getting into 40 meters.
>> Something I cant do with a Dunstar filter between the exciter and
>> amplifier.
>>
>>
>> 73
>> Jim W7RY
>>
>>
>> -----Original Message----- From: Bill Turner
>> Sent: Saturday, March 19, 2016 8:15 PM
>> To: RTTY Reflector
>> Subject: Re: [RTTY] bandpass filters
>>
>> Bandpass filters at the 1500 watt level are quite expensive - around
>> $400 for each band. Do the math.
>>
>> If you want to save some money, you can build your own. Accompanying
>> each ARRL Handbook is a CD which contains design software for high
>> pass and low pass filters. By choosing the cutoff frequencies
>> carefully and putting them in series, you could save a considerable
>> amount of money.
>>
>> This particular software is especially useful because it uses standard
>> value components. In fact that is the name of the program: SVC Filter
>> Designer. Here's a hint: Each filter segment can be looked at as a
>> parallel resonant circuit, i.e. a coil paralleled by two capacitors in
>> series. You can use a grid dip meter to carefully adjust the coil to
>> resonate at the frequency given by the standard equations for
>> resonance. Do each filter segment separately, then connect them
>> together. You can check the final result by sweeping it with an SWR
>> meter while terminated into a dummy load or a real antenna.
>>
>> It is also available at this website: http://tonnesoftware.com/
>> along with other filter design software and there may be others as
>> well. It is well worth browsing around.
>>
>> 73, Bill W6WRT
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>
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