"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" <[email protected]> To: <[email protected]> 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 > > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1424 / Virus Database: 2437/5223 - Release Date: 08/25/12 > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
