Topband: DX Engineering Preselector

[Tom W8JI]

All of this obscures the real issue. The real issue is a device that uses 
back-to-back diodes to limit signals has soft limiting that starts well 
below the clamp voltage. If the advertised clamping voltage is .3 volts, 
that is in the negative dBm range. Most receivers are far into the positive 
dBm range, so the addition of a device like that would just kill the system 
dynamics.

Still..........

>I understand the process quite well but I dont understand the need for such
> a device on a Beverage the large majority of the time. The only time Ive
> used a preamp is on those quietest of nights where only 9-10dB of gain 
> from
> a preamp with a 1.5dB NF brings a ghost of a signal out of the noise.

He clearly said he has an 8 circle array, not a Beverage. The "gain" of an 8 
circle is highly dependent on element spacing. As a matter of fact, what the 
manufacturer sets as sensitivity threshold really controls how small the 
array can be. It's all about destructive phase in the forward direction, 
which gets worse and worse with close spacing.

> I
> didnt have to buy it either as I have the equipment to design, build, and
> test to my own stations needs.

  :-)

>And not a generic catalog item such as the
> RPA-1 which amplifies everything from LF to VHF and dumps it all into your
> receiver including everything from the BCB. No wonder it and the limiter 
> are
> overloaded. The rather generous specs are for a single signal only, not
> thousands of them.

Actually, that isn't true. The IP3 test is NOT a single signal test. It is a 
pretty good test of multi-signal capacity. In any event, despite 
non-techical claims, someone would be hard pressed to generate IMD in the 
amp in any condition where the amp really is needed. Because I have cable 
and splitting losses, I run them all the time on my system here (without 
pre-filters), although I do agree with a short feeder and a normal Beverage 
they probably are not required.

But then he did say he had a different antenna....and none of us know how 
sensitive it really is.  :-)


> At the very least you should have a 160M bandpass filter, or a BCB filter 
> if
> you dont want to bother switching individual filters per band, ahead of 
> the
> RPA-1. Note also the instructions include methods to reduce gain, not all 
> of
> which I agree with.

The instructions are good.

I would not run a bandpass filters ahead of that amp in any normal 
situation. We run multiple transmitters here on site, with NO pre-filtering, 
and the amplifiers are way down the list on limiting things in the system. 
As a matter of fact, we transmit while receiving on the same band without 
issue. We also sometimes have transmitters on multiple bands at the same 
time.

If the amplifier is being hit with a very strong local BC signal it might be 
good to knock out the BC station, but the amplifier will handle far more 
than any receiver. Here is how his system stacks up:

1.) The limiter hard-limits in the negative dBm range, because it supposedly 
clamps at 0.3 volts.

2.) The early element amplifiers run about 10 dBm output TOI

3.) The Hi Z amp in the box is about 30 dBm TOI

4.) The DXE amp is about 45 dBm TOI.

Is it any surprise he started having problems when the limiter was added? I 
haven't found a device yet with back-to-back diodes that does not ruin a 
system's dynamic range. This is because the diodes start to go non-linear 
resistance below the hard-clamping voltage, which is already far too low.

73 Tom