Tim
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@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@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.
>>
>> CONCLUSION
>>
>> 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.
>>
>> BANDPASS FILTERS
>>
>> 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.
>>
>> SO2R WITH A TRI-BAND YAGI AND SINGLE FEEDER?
>>
>> 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
>> _______________________________________________
>> CQ-Contest mailing list
>> CQ-Contest@contesting.com
>> http://lists.contesting.com/mailman/listinfo/cq-contest
>>
>
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|