[CQ-Contest] Bandpass Filter

Timothy Coker n6win73 at gmail.com
Mon Jul 9 06:22:20 PDT 2012

Bob, what are your thoughts on the benefit of switching an individual feed
line interlaced monobander, like the C31XR, over to a single feed line
tribander with an antenna combiner attached?

Assuming one would employ a high power combiner, would coaxial stubs after
the combiner provide enough attenuation in front of the amplifiers? Or must
one use high power band pass filters?


Tim /N6WIN
On Jul 9, 2012 5:57 AM, "Bob Henderson" <bob at 5b4agn.net> wrote:

> An interesting thread.
> A single tri-fed C31XR has been used with significant success in both SO2R
> and M/2 scenarios at P3F for 8 years now.  The single tower installation
> supports a C31XR, an XM240 and an 80m inverted vee dipole. The tower itself
> is shunt fed on 160m.
> More recently a second tower with additional antennas has been added but
> configuration of the first tower remains broadly as it was initially
> configured.
> In any mult-transmitter installation it pays to carry out some kind of
> cross coupling audit before battle commences so as to avoid unwanted
> equipment damage.  Such an audit can become very demanding where precision
> is sought and in these cases will be well beyond the measurement capability
> of most contesters.  Where precision is required this may best be left to
> the techies among us.
> As contesting amateurs what we need are some broad metrics which will
> safely allow us to configure an SO2R or M/M station without risking
> equipment damage or suffering from cross band interference.  We want a KISS
> approach to the issue, consistent with the tools available to the typical
> contesting ham.
> An antenna cross coupling audit which measures the power dumped into a 50R
> load using a simple shack power meter does not come even close to providing
> scientifically valid measurements BUT it nonetheless provides a useful
> measurement metric and one within the capability of any ham to carry out.
> Using this approach when I installed the C31XR 8 years ago the following
> results were found and as I recall published to this reflector at the time.
> *20m driven then 15m -32db and 10m -40db**15m driven then 20m -12db
> and 10m -25db**10m driven then 20m -16db and 15m -32db*
> -12dB dumped into a 50R load attached to the coax feeding the 20m yagi when
> the 15m yagi is driven translates to 100W cross coupled when the drive
> power is 1500W.  It is easy to undermine the validity of this measurement
> on the basis that the receiver switched to 20m and on the end of the 20m
> coax will not present a 50R load to the 15m cross coupled RF.  It can
> further be undermined by drawing attention to the fact that neither will
> the 20m DE source be 50R so far as that 15m energy is concerned.  With
> neither source nor load impedances valid we might as well give up and stay
> single op.
> I don't think so.
> Seeing that your 1500W on 15m can dump 100W into your 50R load at the end
> of the coax feeding your 20m yagi is very useful information.  Impedances
> invalid or not, this is a pretty solid clue that if you are daft enough to
> attach your 20m receiver to this coax you can wave it good bye.  It will
> reply with smoke signals.
> While the measurements detailed above lack scientific validity they
> nonetheless provide a vitally useful insight.
> A simple metric I adopted early on and which has served me well is:
> "No more than 4W cross coupled from any one antenna to any other one using
> the above procedure."  *** This value assumes external BPFs will be used
> between transceivers and associated amplifiers.
> Coaxial stubs are the approach I recommend is used to ensure the 4W figure
> is achieved.
> When it comes to coaxial stubs location of the stub is critically important
> where maximum attenuation of unwanted is required.  Much has been written
> about this elsewhere so I won't dwell upon it here.  However, it is
> questionable whether peak performance of a stub need be achieved.  An
> average ham with a small shack will likely have concerns over just how much
> coax cable he has room to stack up there.  My point being that an
> imperfectly located stub will easily manage the 14dB rejection required to
> drop the hostile 100W down to 4W.  The highest rejection which can be
> achieved is clearly desirable.  Throwing away benefit without good cause is
> indeed folly but let not the pursuit of the perfect become the enemy of the
> well good enough.
> Stubs serve two puposes and your investment in them should take account of
> this.
> 1.  Management of hostile levels of cross coupled fundamental energy.
> 2.  Suppression of harmonics.
> Initially at P3F we used Dunestar 600 multiband BPFs between each
> transceiver and its associated amplifier.  We became dissatisfied with
> these for three reasons.
> 1. We found that even low levels of cross coupled RF could occasionally
> cause welding of the contacts in the small filter select relays.
> 2. We found some filters exhibited high insertion losses.
> 3. We had occasional capacitor failures.
> As a consequence of our experience a 6 band unit based upon 3 pole
> Chebyshev filters (aka W3NQN) was designed with plug-in filters providing
> for a failing unit to be replaced or repaired while leaving the remaining 5
> filters available for use.  Instead of the cheap ceramic capacitors used in
> commercially available units, custom manufactured transmitting mica
> capacitors were specified.  These use very high grade mica and are
> certified iron free.  This is very important in that the presence of iron
> which is found in most standard grade mica capacitors is the root of
> resistive loss and consequently component failure.  The unit was also
> designed to support an internal plug-in BCD band decoder and a set of
> antenna relay matrix drivers so avoiding the cost and clutter associated
> with external decoder/drivers. Detail concerning this BPF unit can be found
> at www.5b4agn.net
> The combination of stubs and external BPFs has provided for a safe and
> interference free set-up compatible with SO2R and M/2 use.
> Is this possible?  Of course it is.  Combatants at WRTC have been doing
> this for some time.  Entrants at the last WRTC in Moscow used BPFs and
> tri-plexers I built to facilitate this.
> Aha I hear you say but they used only 100W radios.  No amplifiers.  Indeed
> so but it is entirely reasonable to consider a tri-plexer to achieve this
> at the 1500W level.  Never mind the distinctly scary level of cross
> coupling in a C31XR with three feeders, how about 2 x 1500W via a single
> feeder to a TH7, TH11, KT34 or whatever tri-bander?  I do this with the
> TH11 we have at P3F on the second tower.  Isolation is such that 1500W on
> 20m results in an only just S9 2nd harmonic on 10m.
> The message I guess is you shouldn't let the science scare you away.  We
> are amateurs we have to be pragmatic in our approaches if we are to achieve
> very much at all.
> 73 Bob, 5B4AGN
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