Hi Eric,
When I first started operating 160 meters, it was at a club station site
which had a full quarter wavelength base insulated tower. We used that
tower for transmit only, because the MWBC QRM was just too much. There
was a 920 kHz and a 1030 kHz 1 kW each transmitter combined into one
tower about two miles away. In the other direction there was a 1150 kHz
station maybe three miles away.
I wanted to use the big vertical for receiving, so I made some series
resonant traps. One for each MWBC transmitter frequency. I used a
spectrum analyzer with tracking generator to get them tuned right on the
BC frequencies. You want the Q high enough so that there is negligible
attenuation at your operating frequency, and the notch at the BC
frequency is really deep, but not so high a Q to make the notch so
narrow that the BC modulation sidebands are no longer in the notch. I
was using capacitances in my series resonant traps of around 2000 to
4000 pF.
The first set of traps I made were using big air variables, paralleled
with doorknob caps and 2" diameter air core inductors. I used these
because they were readily available, and I did not need to worry about
their ability to dissipate any of my transmitter power. Since the traps
are not perfect they will have to dissapate some of your desired
transmit signal (although very little) and some of your transmitters
spurious emissions too. I was using a Kenwood TS-440 which may have
significant phase noise.
Later when I started using an amplifier, I modified the TS -440 to have
a separate RX antenna input, and external TX/RX QSK switching. Then,
since there would be no transmitter power on the dedicated RX antenna
port of the rig, I made a much smaller version of the traps, using
J.W.Miller miniature inductors and dipped silver mica capacitors. I made
another set for a friend. I explained that I did not expect the little
tiny inductors to hold up to transmit power levels. He did not listen,
and proved my suspicion was correct by smoking the inductors.
The series resonant traps are really easy to make. The spectrum analyzer
/ tracking generator really helped to get them tuned just right, and to
know how much rejection to expect, and how much loss on 160 meters. I
was getting at least 30 dB rejection on the MWBC frequencies and only a
couple tenths dB loss at 1800 kHz. I am sure you could do it without the
spectrum analyzer/ tracking generator. I used the reactance charts in an
old ARRL handbook to get the rough capacitance value for the fixed
inductances I was able to buy. Then I added incrementally capacitors
until the notch was right on the MWBC carrier frequency.
A high pass filter is a little bit more complicated. And if you plan on
using it in another location, where the strongest MWBC signals are on
different frequencies, you won't have to retune it. With just traps you
can still do MWBC DX listening elsewhere on the band.
By the way, thanks for the presentation at the Topband Banquet in Dayton.
73 DE N6KB
Eric Scace K3NA wrote:
>Hi Eric --
>
> We have an AM station about 2 miles from the W1KM site. It's small
>but runs 5 kW during the day on 1420... enough to mess up antenna
>analyzers hooked to the low band verticals... and enough to trash receivers.
>
> Fortunately at night it drops to a 1 kW directional pattern and we
>don't see it. But we've been thinking about a filter anyhow so that we
>can chase DX near sunset/sunrise on 160m. I would be curious to learn
>more about the BCI filter you are using.
>
>73,
> -- Eric
>
>on 05 May 28 21:21 Eric Rosenberg said the following:
>
>
>>I've been following the thread on BCI to the Orion with both interest
>>and amusement.
>>
>>I live in Washington, DC proper, 5.5 miles from a 50kw AM station
>>operating at 1500 kHz (signal measured a -10 dBm with a lab-grade
>>spectrum analyzer on my inverted-L), and another relatively high power
>>AMer at 1260 kHz.
>>
>> From my roof I can see (and have photos of) all of Washington's TV
>>(NTSC and HD) and FM broadcast towers, not to mention the plethora of
>>US and foreign government, commercial and other point-to-point
>>transmitters and repeaters that operate into the microwave bands and
>>seemingly beyond.
>>
>>Regardless of the number and size of cavity filters, weak signal and
>>amateur satellite operations are difficult (I used to operate on AO-13
>>and to a lesser degree the pacsats) or impossible.
>>
>>And the intermod/overload on 160 is pretty amazing, regardless of the
>>radio I've used... be it Kenwood, Icom, Yaesu or Ten Tec.
>>
>>For the past 5 years, I had an Omni-6+. For the past year, an Orion.
>>
>>The solution? Well designed and built BCI filters. I spent as lot of
>>time researching what was available in the amateur radio
>>world. Neither the ICE filter (402X), W3NQN, Top 10 Devices or anyone
>>else's worked for me. In the end, I did find one, not (yet)
>>commercially available filter that worked so well for me that I had my
>>best score ever in the CQ 160 contest! If and when it becomes
>>available, I'll post information here on the reflector.
>>
>>A great resource for locating the broadcast (AM, FM and TV) stations in
>>your neighborhood is AMSTNS and TVFMSTNS by Bob Carpenter,
>>W3OTC. Bob's software was invaluable to me in identifying the local
>>broadcasters to better understand who and where and to what degree the
>>offending stations might be. An overview of the software is available
>>at http://users.erols.com/rcarpen/INFO0227.html while the latest
>>versions of the software are available at
>>http://home.earthlink.net/~lvehorn/ For best results follow the
>>various links!
>>
>>The bottom lines is that if you're close to a broadcaster and suffer
>>from intermod and overload problems, don't blame it on the
>>radio! External filters are the only solution.
>>
>>GL!
>>
>>Eric W3DQ
>>Washington, DC
>>
>>
>>
>>
>>
>_______________________________________________
>TenTec mailing list
>TenTec@contesting.com
>http://lists.contesting.com/mailman/listinfo/tentec
>
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|