[Lowfer] Re: IC718 at LF

[James Moritz]

At 04:05 09/04/2003 -0400, you wrote:
>Sounds like what you said makes sense to me. I just cannot hear anything
>other than static or long, unbroken tones at the critical low frequencies
>with my IC-718. What do you recommend for an external pre-amp, keeping in
>mind that I can barely solder, let alone build my own pre-amp...

Dear Jackie, Lowfers

I have an IC718 mostly for HF use - but I have tried it at LF on 136kHz. 
Like most amateur rigs with general coverage RX, LF sensitivity is pretty 
poor, about 20dB down on the sensitivity at HF. Also typical is the front 
end, which for MF and below is just a low-pass filter with an attenuator 
pad. Simply connecting a random chunk of antenna to it will not work very 
well at LF for a couple of reasons. First, any likely antenna will be far 
too small to be self-resonant at LF, and a huge mismatch will exist between 
antenna impedance and RX input impedance, making the already poor 
sensitivity truly dismal. Second, the antenna will be a better match to the 
vastly more powerful MW AM broadcast stations, which will overload the 
front end and cause lots of intermod birdies, which is probably what you 
hear at the moment.

The simplest way to get reasonable LF receive performance with this rig is 
to use a tuned long-wire antenna to improve matching - this can be as 
simple as an adjustable series inductance between the antenna and the RX 
input, which resonates with the antenna capacitance. For example, I use an 
inverted L with a total of around 50m of wire, which has a capacitance at 
LF of about 340pF. At 136kHz, a series inductance of about 4mH tunes the 
antenna to resonance, and gives perfectly adequate sensitivity to hear down 
to the band noise with the IC718. The tuning inductor must be made 
adjustable, because the resonance peak will only be a couple of kHz wide. 
But this also has the major advantage that it filters out most of the MW 
broadcast station signals, and greatly reduces the amount of intermodulation.

An "off the shelf" solution may be to use one of the active whips or loop 
antennas available for LF. The better ones have quite good performance. But 
they may not have enough gain to overcome the poor sensitivity of the rig, 
and being wide-band, will not help in reducing intermods if you live near 
to broadcast stations. Also, any antenna can be affected by local mains 
noise, but that is a different story.....

Cheers, Jim Moritz
73 de M0BMU