[CQ-Contest] Bandpass Filter

Bob Henderson bob at 5b4agn.net
Mon Jul 9 06:54:17 PDT 2012


I'm not sure there would be any significant advantage.  I have no plans to
do this myself as it would require I build another high power tri-plexer.
That's a lot of work just to stand still.  I doubt I would opt for it even
in a new installation even though a coaxial version can negate the need for
stubs, though not for BPFs between transceiver and amp.

As you might imagine insertion loss is a key concern at 2 x 1500W.

73 Bob, 5B4AGN

On 9 July 2012 13:22, Timothy Coker <n6win73 at gmail.com> wrote:

> 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?
> 73,
> 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
>> 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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