Topband: DX Engineering Preselector

[ZR]

"He clearly said he has an 8 circle array, not a Beverage."

Yes, John reminded me of that a day ago.

 So when are you going to publish the schematic and parts list for that 
preamp that you use?

Many of us are still capable of building.

Cal
KM1H


----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: <topband at contesting.com>
Sent: Saturday, August 25, 2012 2:29 PM
Subject: Re: Topband: DX Engineering Preselector


> 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
>
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