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