Hi Joe, I placed my watt meter to read power backwards in between my
antenna and dummy load. When placing the filters inline I placed them
between the watt meter and antenna. This is my understanding of how to test
with very simple equipment that most of us have. More accurate measurements
can be taken with equipment that I do not own or understand how to use :)
That is where guys like Tom and others come in.
73,
Tim / N6WIN.
On Wed, Jul 4, 2012 at 5:49 AM, Joe <nss@mwt.net> wrote:
> Wow,
>
> This has me curious. How did you measure the power coming back down the
> other feedline? Sounds like a simple question, Is it as simple as a
> wattmeter in line and dummy load?
>
> Joe WB9SBD
> Sig
> The Original Rolling Ball Clock
> Idle Tyme
> Idle-Tyme.com
> http://www.idle-tyme.com
> On 7/3/2012 5:06 PM, Timothy Coker wrote:
> > 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@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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