KX3 - Does It need Preamp for 6M?

[Edward R Cole]

Wayne cites the MDS is -140 dBm with the KX3 preamp set to
30-dB.  That is excellent and all that is needed for terrestrial 6m
operation.

I would dispute the advantage of mast mounting a preamp on 6m.  I
recently showed a reader what the effect would be using a 0.5 dBNF
25-dB gain GasFet preamp at the antenna vs at the radio in an eme
operation.  Sky noise temperature runs > 2000K at 6m even for quiet
locations with the result of only increasing MDS by a tenth of a dB
when the preamp is located at/near the antenna.  Use of low-loss coax
like Belden-9913 or Times LMR-400 is sufficient to run 6m preamp at the radio.

On frequencies 2m-and-up the sky noise situation changes this to
where there is marked improvement locating a preamp at the antenna
(and essential for weak-signal work like eme).  I will be running
800-1000w on 6m-eme later this year (after the yagi is mounted on a
new tower) and will run my 6m GasFet preamp at the K3 using the RX input.

73, Ed - KL7UW
http://www.kl7uw.com
[hidden email]
"Kits made by KL7UW"

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[Jim Brown-10]

On 6/20/2013 10:25 AM, Edward R Cole wrote:
> I would dispute the advantage of mast mounting a preamp on 6m.

It all depends on how much loss is in your run of coax, and how quiet
your location is.  The run to my SteppIR is 350 ft of 7/8-in hard line,
just under 1 dB at 50 MHz, and my QTH is not very quiet, so I'm happy
with my ARR GasFET preamp at the K3 patch point.

The decision to mast-mount or not is pretty simple.  If you hear the
band noise increase by at least 7 dB when you connect the antenna, leave
the preamp in the shack.  If the noise rises by only a few dB, compute
the loss in your line (see mfr data for the coax you're using).  If the
noise increases by only 3dB with the connected (and no preamp), you'll
see an increase in your ability to hear weak signals of roughly one
third of the line loss. If the noise doesn't increase at all, you'll see
in improvement equal to ALL of the line loss.

How much is a dB worth?  It all depends how close to the noise level the
signal is that you're trying to copy.  If it's VERY close to the noise,
a dB or two can be the difference between making and missing the QSO.

73, Jim K9YC
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