Strong Local FM Broadcast / MFJ 259b
 

The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

Strong Local FM Broadcast / MFJ 259b
 

"dykesc" wrote in message
...
The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

One way to find out. Measure the SWR through a low pass filter that is meant
to be used at a transmitter output. These things usually start to roll off
at about 40 MHz, and will have around 80 db loss at 100 Mhz.

Tam/WB2TT

Strong Local FM Broadcast / MFJ 259b
 

dykesc wrote:
The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

Absolutely. Ironically, among the many uses for an "antenna analyzer",
the one they're least suited for is analyzing actual antennas -- at
least in a lot of urban settings. What I've done is constructed a number
of "half wave filters" which effectively filter out TV and FM signals
without affecting the measured impedance appreciably on the band they're
designed for. You do need one for each band, however.

A "half wave filter" has a unity voltage, current, and impedance
transformation, and a 180 degree delay, at the design frequency, like a
half wave transmission line, but provides a lowpass filtering function.
They're broad enough that you can use one over a whole band without too
much impedance error. It consists of a shunt C - series L - shunt C -
series L - shunt C, with the reactance of every element except the
middle C equal to 50 ohms (to mimic a 50 ohm transmission line). The
reactance of the middle C is half the value of the others -- you can
conveniently use two capacitors in parallel, each having the same
reactance as the end capacitors. I generally use mica or NPO ceramic
capacitors, and inductors wound on type 6 or type 2 powdered iron cores,
for HF. You should test the filter by measuring several known impedances
with it in series with the impedances, and with it removed.

You can make a highpass version, too, by exchanging the Ls and Cs. In
your case, though, you need lowpass ones.

Roy Lewallen, W7EL

Strong Local FM Broadcast / MFJ 259b
 

Roy Lewallen wrote:
dykesc wrote:
The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

Absolutely. Ironically, among the many uses for an "antenna analyzer",
the one they're least suited for is analyzing actual antennas -- at
least in a lot of urban settings. What I've done is constructed a number
of "half wave filters" which effectively filter out TV and FM signals
without affecting the measured impedance appreciably on the band they're
designed for. You do need one for each band, however.

A "half wave filter" has a unity voltage, current, and impedance
transformation, and a 180 degree delay, at the design frequency, like a
half wave transmission line, but provides a lowpass filtering function.
They're broad enough that you can use one over a whole band without too
much impedance error. It consists of a shunt C - series L - shunt C -
series L - shunt C, with the reactance of every element except the
middle C equal to 50 ohms (to mimic a 50 ohm transmission line). The
reactance of the middle C is half the value of the others -- you can
conveniently use two capacitors in parallel, each having the same
reactance as the end capacitors. I generally use mica or NPO ceramic
capacitors, and inductors wound on type 6 or type 2 powdered iron cores,
for HF. You should test the filter by measuring several known impedances
with it in series with the impedances, and with it removed.

You can make a highpass version, too, by exchanging the Ls and Cs. In
your case, though, you need lowpass ones.

Roy Lewallen, W7EL

He's getting RFI at one frequency. I was thinking an open quarterwave
coaxial stub tuned to 100.5 MHz would serve to trap the offending signal,
and be easy to implement. Cut it a tad long from the calculated length,
connect it to the analyzer input using a tee adapter, and "tune" it up in
frequency using a pair of wire cutters:

246/100.5 = 2.45' (or 2' 5-3/8") * Vf

Bryan WA7PRC

Strong Local FM Broadcast / MFJ 259b
 

He's getting RFI at one frequency. I was thinking an open quarterwave
coaxial stub tuned to 100.5 MHz would serve to trap the offending signal,
and be easy to implement. Cut it a tad long from the calculated length,
connect it to the analyzer input using a tee adapter, and "tune" it up in
frequency using a pair of wire cutters:

246/100.5 = 2.45' (or 2' 5-3/8") * Vf

Bryan WA7PRC

What will that stub look like above and below the notch frequency? It sure
won't be invisble.
eg: 50 Ohm cable is around 27pF/'.
Dale W4OP

Strong Local FM Broadcast / MFJ 259b
 

On Sun, 30 Mar 2008 18:34:41 -0700 (PDT), dykesc
wrote:

The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

The worst is yet to come:
More transmitters are likely to be added to the new tower...

John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

Strong Local FM Broadcast / MFJ 259b
 

"Roy Lewallen" wrote in message
...
dykesc wrote:
The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

Absolutely. Ironically, among the many uses for an "antenna analyzer", the
one they're least suited for is analyzing actual antennas -- at least in a
lot of urban settings. What I've done is constructed a number of "half
wave filters" which effectively filter out TV and FM signals without
affecting the measured impedance appreciably on the band they're designed
for. You do need one for each band, however.

A "half wave filter" has a unity voltage, current, and impedance
transformation, and a 180 degree delay, at the design frequency, like a
half wave transmission line, but provides a lowpass filtering function.
They're broad enough that you can use one over a whole band without too
much impedance error. It consists of a shunt C - series L - shunt C -
series L - shunt C, with the reactance of every element except the middle
C equal to 50 ohms (to mimic a 50 ohm transmission line). The reactance of
the middle C is half the value of the others -- you can conveniently use
two capacitors in parallel, each having the same reactance as the end
capacitors. I generally use mica or NPO ceramic capacitors, and inductors
wound on type 6 or type 2 powdered iron cores, for HF. You should test the
filter by measuring several known impedances with it in series with the
impedances, and with it removed.

You can make a highpass version, too, by exchanging the Ls and Cs. In your
case, though, you need lowpass ones.

Roy Lewallen, W7EL

I tried a 2.5 MHz HPF with an MFJ analyzer to measure a 75 m antenna.
Unfortunately there is too much phase shift. Really not usable below about 5
MHz. The MFJ works fine during daytime hours before all the signals come up.

Tam/WB2TT

Strong Local FM Broadcast / MFJ 259b
 

Tam wrote:

I tried a 2.5 MHz HPF with an MFJ analyzer to measure a 75 m antenna.
Unfortunately there is too much phase shift. Really not usable below
about 5 MHz. The MFJ works fine during daytime hours before all the
signals come up.

The "half wave" filter I described makes no impedance disturbance only
at the frequency at which all the components have the same reactance.
The cutoff frequency, as I recall, is close to that frequency. So you'd
have to make a filter with a cutoff close to 3.8 MHz, not 2.5 MHz, in
order to measure a 75 m antenna if you're using this kind of filter.
There are, of course, many other types of filters, but just about any
will affect the impedance measurement except perhaps over a fairly
narrow range of frequencies.

Roy Lewallen, W7EL

Strong Local FM Broadcast / MFJ 259b
 

On Mar 31, 11:32 am, Roy Lewallen wrote:
Tam wrote:

I tried a 2.5 MHz HPF with an MFJ analyzer to measure a 75 m antenna.
Unfortunately there is too much phase shift. Really not usable below
about 5 MHz. The MFJ works fine during daytime hours before all the
signals come up.

The "half wave" filter I described makes no impedance disturbance only
at the frequency at which all the components have the same reactance.
The cutoff frequency, as I recall, is close to that frequency. So you'd
have to make a filter with a cutoff close to 3.8 MHz, not 2.5 MHz, in
order to measure a 75 m antenna if you're using this kind of filter.
There are, of course, many other types of filters, but just about any
will affect the impedance measurement except perhaps over a fairly
narrow range of frequencies.

Roy Lewallen, W7EL

Since there is just one frequency that's causing trouble (apparently),
and since I gather that it's removed in frequency a pretty good
percentage from the frequencies of interest, it may be possible to
block it with a fairly sharp notch filter. As Roy notes, the 100.5MHz
FM signal may not be the only one causing trouble, but assuming it is,
try this: shunt across the input to the analyzer, place a series LC.
The L: 9 turns #12AWG bare copper, 0.75" ID, 1.5" long (about 750nH,
SRF about 150MHz). The C: a high quality 1-5pF trimmer capacitor.
I'd recommend a good piston trimmer, but they tend to be expensive
unless you happen to have some or can get some surplus. Depending on
the exact coil characteristics, the cap will adjust to a 100MHz
resonance at about 2pF. By keeping a high L:C ratio, the notch will
be sharp--not as deep as it could be with lower L:C, but it should be
deep enough if you use good construction practice. The suggested
values should give you at least 25dB attenuation at the notch
frequency, and should look practically like the capacitor value (about
2pF) at HF.

Cheers,
Tom

Strong Local FM Broadcast / MFJ 259b
 

"Tam" wrote:
The MFJ works fine during daytime hours before all the signals come up.
_________

Unless (as in the OP) the source of the interfering signal is a
line-of-sight VHF/UHF broadcast station -- the nearby fields of which can be
very high, and little affected by the time of day.

RF