Roger: I tried to post a reply to the group prior but it did apparently
did not make it past the admin monitor or it exceeded message size
limit.
I found the key to getting the FT-817 to receive well on LF and MF is to
use a low pass filter with good in-band return loss characteristics between
the antenna (e-probe, EWE, etc.) and the
FT-817. If you have ever tried listening to an AM broadcast station
with the FT-817 and it's VHF rubber duck antenna, you know it is stone-cold
deaf that way. But if you use an antenna with a reasonable 50 ohm match
at lower HF, the FT-817 hears AM band stations (530 ~ 1600 kHz) very well. The
same principle applies to LF/MF in the 100kHz to 500 kHz range. The
FT-817 receiver front end needs to see something close to 50 ohms to receive
signals efficiently. But it also needs most of the AM band energy
rejected so it will not overload. Using an LPF with a good pass band
match seems to solve the problem when using an e-probe antenna at LF/MF with
the FT-817. Also do not use the preamp in the FT-817. Turn off the AGC
and NB.
There is a VE7BPO NDB filter with build information online that is easy
to construct with commonly available parts if you do not have an LF/MF LPF
filter available. I'm sure if you try it or another good LF/MF LPF with your
FT-817 you will see an immediate order-of-magnitude improvement in receive
performance at LF/MF. I use a variation of the VE7BPO filter for 630m
and a 9th order 300 kHz Butterworth LPF for LF. The Butterworth LPFs
have very good pass-band return loss and the FT-817 responds very well to
having the filter between the antenna and the radio at LF.
For even better performance I run a broadband LF/MF 2N3904 preamp after
my LPF and either a 3dB pad or a 50 ohm universal diplexer for the frequency
of interest after the preamp and into the FT-817. I built the 2N3904
preamp last night to try it out based on your recent FT-817 LF
postings...
Last night I was able to decode WH2XXP in AZ from PA with the FT-817 at
some 3000 km distant 55 times with the FT-817 on the 2200m band. I was just
able to occasionally decode WH2XXP with only the LPF and no post filter
preamp. With the 2N3904 preamp into a 3 dB pad and then into the FT-817
I was able to obtain a best overnight SNR of -18 on WH2XXP at 3000 km distant.
I did need to use the attenuator in the FT-817 to adjust the rx level.
Quick sketch of preamp and FT-817 rx layout...
Pix of filter-preamp setup but with 137 kHz diplexer on preamp
output instead of 3 dB pad...
73 Mike wa3tts
