Your first thought might be that 180KHz is not enough to cover the 80/75M band, but remember that this preselctor is tunable, so you can put the peak wherever you want it. At 3.5MHz, in the “Lo” position, the insertion loss at the peak is 10.27dB, the passband ripple a mere 0.83dB, and the bandwidth at the -3dB points 180KHz. With the switch in the “Hi” position, at the highest frequency setting, which is 15.6MHz in my unit, the insertion loss at the peak is 4dB, the passband ripple 1.75dB, and the bandwidth at the -3dB points is 1.66MHz. With the switch in the “Lo” position on my preselector, at the maximum frequency setting (lowest capacitance), the peaks were spaced 5MHz apart, and the difference between the dip and the two peaks was 14dB – far too much. The switch needs to be flipped to the high position over about 7 or 8MHz, otherwise the response is far too broad. Switching in the extra 10pF reduced the insertion loss to 10.5dB which, although still a little high, is a lot better. Without it, the insertion loss at the peak of the response is a whopping 32dB. For the lower frequencies, an extra 10pF capacitor is switched in. I’m thinking that some coupling is happening inside the polyvaricon perhaps? A value of 1pF gave an acceptable response curve above about 8MHz. The only reason I can think of for this, is that the separate gangs in the polyvaricon are not as well isolated as they would be in a larger air-spaced part. The final value of 1pF was far lower than I had expected. I began with higher values, of 47pF and then 39pF, but found that the coupling was too tight, and I ended up with 2 distinct peaks in the response, spaced far apart, with a large dip in between them. The 1pF capacitor that couples the two tuned circuits might seem rather low. I wanted these coils though, for the ability to easily adjust the inductance. It is possible to wind a similar coil on a toroid. Some of the coils in this series are direct replacements for the Toko KANK series, which were popular with UK homebrewers in the past. If you are not a GQRP Club member, these coils are available from Spectrum Communications in the UK, who also sell on eBay. The L denotes that the secondary is a low impedance winding, suitable for matching to 50 ohm systems. The nominal inductance is 5.3µH, though it is adjustable over a fairly wide range. Also supplied, are the inductors with adjustable ferrite cores. They come with a bolt and plastic spindle, for attaching a knob. GQRP Club member sales can supply the polyvaricons (to members) with a mounting kit that consists of 2 different lengths of mounting screw, to allow for different thicknesses of front panel. I think it would be possible to find a suitable air-spaced part that would fit into this space but, for the sake of timeliness, I plumped for a polyvaricon. Although my preferred variable capacitor of choice would have been an air-spaced component, I wanted to fit this into the same LMB Heeger 143 enclosure that I used for the other two receiver modules, the DC receiver mainframe, and the Si5351 “VFO”. The circuit is a very straightforward and standard double-tuned bandpass filter. A tunable passive HF preselector seemed like a good way to get this particular show on the road. AM reception is not great on a direct conversion receiver, but there are quite a few non-ham SSB and CW signals to listen to outside the ham bands, and it would be good to be able to do that on this receiver. That may happen, but I also wanted to listen in between the amateur bands. I purchased a 10-pack of these filter kits from QRP Labs, intending to, at some point, assemble most of them for listening to the amateur bands on this little receiver. The first BPF I constructed was for 40M, and it did a fine job of removing many of the spurious responses I was experiencing with no antenna filtering in circuit. It was constructed so that the bandpass filters from QRP Labs could be plugged in, but this circuit section could be constructed from scratch, if desired. The mainframe includes a spot for an onboard plug-in bandpass filter. However, the outboard functions of local oscillator and antenna filtering can swapped around and changed at will. The little box that contains the DBM, diplexer, and AF amplifier that make up the mainframe will most likely stay largely the same, now that they are built and boxed up. The intention was more to have it as part of an experimental platform. You can see other info at the WCA's historical world record list.When I built the VE7BPO DC Receiver Mainframe recently, it wasn’t intended to end up as a final finished project. This is a list of all WCA World Records for the 4x4x4 event, with links to videos, discussion threads, and reconstructions when possible. Take care about information on this page and update it if you know more recent information. This page may be currently outdated (since world records may occasionally change).
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