[CQ-Contest] Bandpass filter

Timothy Coker n6win73 at gmail.com
Wed Jul 4 08:31:27 PDT 2012

Let the flames begin, LOL.

Sorry Keith, the measured isolation between 15m TX and 20m RX on the C31XR
with physical separation of only 18" is legitimate. Now do consider that I
am only an average ham using a Kenwood SW-2000 that specs to +/- 10%
accuracy. You are right though... 135w was excessive, it was 125w +/- 10%.
Natan from the old F12 even sent me a response about this issue in 2002
stating that the back fed power would be over 100w depending on band
combination. I found that Ice 419As with no stubs would not burn out under
those conditions while running SO2R as a very novice contestor (A better
SO2R operator would probably burn them up because they would transmit on
both rigs more often than I was able to). Now that I am wiser I have
invested in both stubs and higher overhead band pass filters.

Now that I am not writing from my vehicle I have the benefit of my test
measurements available to me. Let me give a more accurate accounting of my

C31XR individual feed lines 15m 1500w TX and 20m RX:

No filters: 125w back fed power (-10.8dB)
W3NQN 20m RX: 1w back fed power (-63.52dB)
23' open stub 20m RX: 1w back fed power (-63.52 db) This level of
attenuation I question and a more accurate test meter should have been used.
23' shorted stub 15m TX and W3NQN on 20m: 1w back fed power (-63.52dB) No
additional isolation with the 15m TX stub.

*23' open stub AND W3NQN on 20m RX: 0w back fed power (>-80dB)

On the other band combinations I measured (without filtering) between
-17.9dB to -28.8dB

Based off the QST article a combiner with band pass filters would help
those of us with driven elements spaced so closely on a single boom and
that had the ability to switch to a single feedline (like the C31XR). From
my own testing even the Inrad combiner will not always provide greater
attenuation as compared to the closely spaced driven elements on an antenna
like a C31XR. However on 15m TX and 20m RX the combiner would give me an
additional 20.6dB of isolation and is something to consider for reducing
the back fed power on that band combination.

When I TX on 20m and RX on 15M I have measured isolation at -27dB. The
Inrad combiner was measured by the ARRL at -26.9dB. Here is a band
combination that would make me no better off.

Those of us wanting to run high power and use a single feedline with
combiner would need to invest in high power filters for their higher bands.
4O3A seems to be the only person offering such a combination.

On 40m 1500w TX with an 80m RX, feed points spaced at ~15' distance, I find
2w back fed power (-28.8dB). Used was a XM240 at 68' and an 80m inverted-v
@53' on the same tower. Antenna spacing is obviously king here... but guys
like myself don't always have that luxury when it comes to wanting to fit
six contest band monobanders on a residential lot.

Let me remind the flamers that I never sent a flame at Tom. I made mention
that I know he is a damned smart guy. I simply read his numbers without
full understanding. I have since come to learn new information from him.
Notice how right Tom was about the 15m TX stub doing nothing and how wrong
I was going off memory while posting half correct information away from my
findings spreadsheet. But my findings about the back fed power does indeed
not take a rocket scientist to figure out... only an average ham with
sub-par measuring equipment.


Tim / N6WIN.

On Wed, Jul 4, 2012 at 7:52 AM, Keith Dutson <kdutson at sbcglobal.net> wrote:

> Does not take rocket science to question your measurement of 135 watts
> feedback power.  About the only way this could happen is if the two
> antennas
> were physically attached together.  And that is even questionable based on
> different bands being measured.  I think there is a flaw or
> misrepresentation in your measurement.
> Keith NM5G
> -----Original Message-----
> From: cq-contest-bounces at contesting.com
> [mailto:cq-contest-bounces at contesting.com] On Behalf Of Timothy Coker
> Sent: Tuesday, July 03, 2012 5:07 PM
> To: Tom W8JI
> Cc: cq-contest at contesting.com
> Subject: Re: [CQ-Contest] Bandpass filter
> I recently measured the power coming back down my 20m line while
> transmitting on 15m. It's over 135 watts of power when using my K3 and Acom
> 2000 at 1500w forward output. Not milliwatts...
> A W3NQN BPF on the 20m coax, placed before the wattmeter and dummy load was
> able to knock the measured power down to 1w of back fed power. A TX stub
> then placed on my 15m TX line then resulted in less than 1W being back fed
> on the 20m line.
> I know you're a smart guy Tom, but not sure where your numbers come from.
> My station QTH has room for one tower and my 20-10m antenna is a C31XR with
> individual feedlines. I cannot go without good filters and expect my K3
> front ends to survive.
> 73,
> Tim /N6WIN
> On Jul 3, 2012 8:02 AM, "Tom W8JI" <w8ji at w8ji.com> wrote:
> > > My understanding is that the main function of my 40m BPF (when
> > > thinking
> > of
> > > my second radio on 20m) is to reduce the second harmonic on 20m
> > > produced by my 40m TX.
> >
> > Nearly all modern radios have excellent harmonic suppression, while
> > external power amplifiers do not.
> >
> > If we have a second harmonic issue, a critically located stub or
> > stubs, or a trap, is far more effective and less costly than a high
> > power BPF.
> >
> > The distance from the source and load, and the characteristics of the
> > suppression device, source, and load, usually greatly affect the null
> > depth of any suppression. For example optimum stub placement from a
> > pi-L network is quite different than optimum placement from a pi network
> or T network.
> > It's different for every antenna, too.
> >
> > The tradeoff is always cost and work, and immunity to system
> > characteristics. A great big expensive filter can work well no matter
> > where it is placed in the line. A cheap system, like a stub, might not.
> >
> >
> > >So if I am on 7.025 running, I will create a very strong second
> > >harmonic signal on 14.050. Depending on my TX power, antenna
> > >proximity,  and  antenna polarization, that signal might be enough to
> > >couple enough power  on  my 20m line to fry my second radio front
> > >end.
> >
> > I doubt that would happen. Most amps are -46dBc or more. With 1500
> > watts, that's about 40 milliwatts. 40 milliwatts will never fry a
> > receiver. Added to that suppression, we have coupling losses between
> > the antennas that are likely 20 dB or more.
> >
> > The only reason to add a filter or stub is if something is out of
> > spec, or someone closeby wants to operate right on top of the second
> > harmonic. For example, I can hear, and be bothered by, harmonics that
> > are only 100 microwatts from hundreds of miles away, **IF** I happen
> > to operate on that harmonic.
> >
> > Of course that level is thousands of times less than a level that
> > could be damaging to another local receiver.
> >
> > The real function of a bandpass on transmitting is mitigating problems
> > from someone, local or distant, operating near the harmonic.
> >
> > If there are issues with TX noise bothering our receivers on an
> > adjacent band except near the harmonic, it is worth looking closely at
> the
> gear.
> > Something is wrong, beyond lacking an extra filter.
> >
> > At even 30-40 dB of external TX antenna filtering, cabinet and line
> > cord leakage will usually dominate harmonic and spurious radiation
> > into the RX antennas.
> >
> > 73 Tom
> >
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