Topband: Re: Receiver

[Tom Rauch]

> >Although the dynamic range of my Omni 6 is fairly good, I've been
> >suffering lately when my neighbor 2 miles to the North is active on Top
> >Band. 
> 
> Hi Barry,
> 
>         There is no commercial receiver that can handle this problem well
> given the dynamic range of the 160 signals you are experiencing (probably
> 70 or 80 dB above S9 for your neighbor to S0 for weak DX).  Having lived
> <2 miles from W0YK and <5 miles from K0RF when I was in Colorado, I can
> assure you that I know what you are experiencing!

A large amount of this problem is also in transmitters. On SSB, 
IMD in modern transceivers is typically only in the -25 to -35 dB 
range, and products extend out many orders compared to older 
rigs that were often immeasurable beyond the 5th or 7th order 
product.

An old Drake T4XC, in proper shape, measures -40 dB below PEP 
wile a KWM-2 I have measures -55 on the third order product. Both 
have IMD into the noise floor higher than the seventh order (which 
would be about 20 kHz away). I can measure IMD out to the 
thirteenth order in many solid state rigs.

Virtually all receivers have dynamic ranges much higher than their 
transmitter IMD suppression.

If you doubt how rigs have deteriorated, just look at QST reviews. 
Remember to subtract 6 dB to get the commercial test standard of 
dB below one tone of a two tone test, and keep in mind the older 
tube rigs QST reviewed years ago were tested at a different 
standard, they used the commercial spec of dB below one tone.

Couple that with synthesizer noise and harsh keying or VCO 
"swooping" when working split, and it can be a problem on CW 
also. Of course those with noisy VCO's will enjoy reciprocal noise 
when listening to weak signals near very strong signals on receive!

>         The solution is to reduce your neighbor's signal before it gets to
> your RX.  The MFJ-1025/6 is perfect for this application since your
> neighbor's amplitude and phase is relatively fixed and can be nulled with
> the Noise Canceler before it reaches your reciever.  You may want to tell
> your neighbor about this because I'm sure he is having the same problem.
 
I'm not sure how the 1025/6 handles this. I asked for some current 
sample units from MFJ, and I'll run some IMD and gain tests on 
them over the next few weeks and post the results, good or bad. 
My "MFJ unit" are homebrew units that were the core of the design 
being sold commercially, but they have higher dynamic range and 
lower noise amplifiers using more expensive components. (The 
FET's cost almost 25 dollars each! ) If the 1025/6 is better than 
most receivers for overload, it would surely help a lot on 
groundwave signals.

It sure would be nice if we could go back to split operation for DX 
and remember to keep SSB as far from weak signals as possible, 
since it appears impossible to get manufacturers to improve our 
radios.
73, Tom W8JI
w8ji@contesting.com



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