I bought an MFJ-269 antenna analyzer with some 160 meter antenna design in
mind. It works fine on my VHF and UHF antennas, but the local 1350 AM
station (which is just a half mile away) interferes when an HF antenna is
connected. The SWR reads 31 and the two analog meters dance to the
station's audio. I tried a home made 1350 KHz notch filter I've used with a
few receivers. The notch filter gets rid of the offending signal, but makes
the 160 and 80 meter readings bad (verified with a dummy load).

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Luckily my Kenwood TS-570 is able to contend with the AM station.

--
Bob D. ND9B

Bob,

MFJ makes something to do that, called the MFJ-731. I live near the
WSM-AM tower, and definitely can "see" them when I do HF work. I am
primarily a VHF-UHF operator, so I can't give you more than a cursory
report. The MFJ-731 seems to take care of the problem without
introducing significant inaccuracies.



--
Alan
WA4SCA

On Sat, 8 Sep 2007 18:16:12 -0400, "Bob D."
wrote:

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Hi Bob,

Call the station, ask for the Engineer. Then ask when they go off the
air for testing. You may be up late into the wee hours, but for only
10 minutes or so.

If you are going to filter the AM out, you have to look through that
filter to see the antenna. Your experience has already borne out the
futility of that. However, I've seen just such advice in the past
with recommendations of charting things through a Smith Chart. I
can't say I have much confidence in that outcome - still, it may beat
out the alternative of waiting a month for the scheduled down time.
Even then, I would still get up that night and get the benchmark.

73's
Richard Clark, KB7QHC

On Sat, 8 Sep 2007 18:16:12 -0400, "Bob D."
wrote:

I bought an MFJ-269 antenna analyzer with some 160 meter antenna design in
mind. It works fine on my VHF and UHF antennas, but the local 1350 AM
station (which is just a half mile away) interferes when an HF antenna is
connected. The SWR reads 31 and the two analog meters dance to the
station's audio. I tried a home made 1350 KHz notch filter I've used with a
few receivers. The notch filter gets rid of the offending signal, but makes
the 160 and 80 meter readings bad (verified with a dummy load).

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Luckily my Kenwood TS-570 is able to contend with the AM station.

If you're open to getting another analyzer here's one to consider. I
have used my AIM 4160 at a friends location which is about ½-mile from
a local broadcast station and it worked fine for 160-metrer antenna
adjustments -although the local BC station runs only 1 KW.

Here's a link showing what you can expect using the analyzer for your
situation.

http://w5big.com/AIM4170_SignalRejection.htm

Danny, K6MHE

I have the BC filter for the MFJ259... Never could make heads or tails
of the so called 'instructions'...
I find that using EZNEC for getting initial element sizes and resonant
frequencies, then using a grid dip meter followed by a transmitter and
an RF current meter for final tuning has always worked for me...
I do use the MFJ259 quite a lot, but for my 80 and 160 arrays the
voltages from rectified BC getting into the analyzer often screws it
up... My best time is middle of the day when there is less propagation
of BC signals - usually that is when I am at the office, however
sigh...

denny / k8do

"Bob D." wrote in message
m...
I bought an MFJ-269 antenna analyzer with some 160 meter antenna design in
mind. It works fine on my VHF and UHF antennas, but the local 1350 AM
station (which is just a half mile away) interferes when an HF antenna is
connected. The SWR reads 31 and the two analog meters dance to the
station's audio. I tried a home made 1350 KHz notch filter I've used with a
few receivers. The notch filter gets rid of the offending signal, but makes
the 160 and 80 meter readings bad (verified with a dummy load).

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Luckily my Kenwood TS-570 is able to contend with the AM station.

--
Bob D. ND9B

MFJ sells a tunable band pass filter, which you would tune to 160 m. By the
way, a high pass filter won't work either. I built a 2 MHz high pass filter
to use on 75 m, but it affected all readings below about 5 MHz. My AM
stations aren't real close; so, the 269 works OK on 75 m during the middle
of the day.

Tam/WB2TT

One kind of filter you can use with antenna analyzers is a "half wave"
filter. This is an LC ladder network which has the delay and impedance
transformation properties of a half wavelength transmission line *at the
design frequency*, so the load impedance appears at the filter input. It
can be made with either a lowpass or highpass frequency response, with
the attenuation beginning not far from the design frequency. While it's
not by any means the sharpest cutoff filter you can build, it has
respectable attenuation beyond the cutoff frequency. It will alter the
impedance if you get very far from the design frequency, but for some
bands you can probably get by with a single filter, provided that the
impedance you're measuring isn't too far from the filter Z0 (see the
next paragraph) -- you should check with known loads at the frequency
extremes you want to measure. The cutoff might not be adequate for the
160 meter/1350 kHz problem at hand. You can cascade more than one for
better attenuation, but then the frequency range limits become more
restrictive and the filter construction more critical.

A "half wave" filter is made by cascading two "quarter wave" filters. A
"quarter wave filter" is simply a pi network whose elements all have a
reactance of Z0, where Z0 is the impedance of the transmission line
which the network imitates. 50 ohms is of course the most commonly
chosen value, but you'll get more extended frequency operation if you
choose a Z0 which is fairly close to the impedance you're measuring. For
a lowpass, use inductors for the series elements and capacitors for the
shunt elements. For a highpass, use the opposite. When you cascade two
"quarter wave" lowpass filters, you have a C-L-C-L-C
(shunt-series-shunt-series-shunt) configuration, where the middle C is
twice the value of the end Cs. A highpass looks like L-C-L-C-L, where
the middle L is half the value of the end Ls.

The inductor Q has to be decent, but not spectacularly good. I use type
6 powdered iron cores for the HF range. Type 2 might be a little better
for MF. Because of the low loaded Q, these filters are quite forgiving
about construction and small value variations. Give one a try -- they're
easy to build.

I live in a high-level RF environment, especially from TV transmitters.
I use my antenna analyzer for a lot of different things -- just about
everything except analyzing antennas, in fact, which ironically is the
one thing it's most poorly suited for. When I really need to know an
antenna's impedance (at least up to 60 MHz), out comes the GR bridge,
with an old Icom R1 receiver as a tuned detector.

Roy Lewallen, W7EL

In message , Bob D.
writes
I bought an MFJ-269 antenna analyzer with some 160 meter antenna design in
mind. It works fine on my VHF and UHF antennas, but the local 1350 AM
station (which is just a half mile away) interferes when an HF antenna is
connected. The SWR reads 31 and the two analog meters dance to the
station's audio. I tried a home made 1350 KHz notch filter I've used with a
few receivers. The notch filter gets rid of the offending signal, but makes
the 160 and 80 meter readings bad (verified with a dummy load).

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Luckily my Kenwood TS-570 is able to contend with the AM station.


I've read some of the other replies, and some of them seem to suggest
unnecessary complexity in obtaining a solution.

First, what does your 1350kHz notch filter consist of? Let's assume it's
a simple series-resonant L/C to ground across the TS-570 RF input/output
(or somewhere across a 50 ohm coax transmission line between a
transmatch and the TS-570 RF input/output). If the L/C ratio is low
(smallish L, biggish C), the notch will be relatively broad, and
relatively deep. If the L/C ratio is high (biggish L, smallish C), the
suck will be relatively narrow, and not so deep (due to the additional
losses in the L). Either way, the notch may be broad enough to pull down
signals at 1800kHz.

At 1800kHz, the notch circuit will appear as an inductor across the
TS-570 input/output. If the reactance is not considerably more then 50
ohms (ie effectively an RF choke), you will lose signal level on
receive. On transmit, it will cause a poor SWR (looking out of the
TS-570). What you need to do is to sharpen up the notch on the LF side.
All you have to do is to parallel-tune the effective inductance of the
notch (at 1800kHz) by adding another capacitor across the trap (not just
across the L). You tune this capacitor so that, at 1800kHz, the trap
circuit appears a parallel-tuned circuit (infinite impedance) across the
TS-570 RF output/input. There should now be very little loss of signal
level, and a good SWR. Obviously, you may need to re-tune the capacitor
slightly as you QSY across the 160m band. On 80m, a much lower value of
capacitor will be needed.

Does this explanation seem to be applicable to your situation, or do you
have something different, which requires a different solution?
--
Ian