Radios Of The 2010 First Final Farewell DX Contest
Dan Denapoli, Open Class and below BCB Class
Chris T, Hobby Class
Dave Schmarder, Active Device Class
Click here to go to my website and see what I did this year:
-Single 7C5 Loctal tube regenerative radio with 6 volt filament and 25 volts DC plate
-Two tuned circuits using toroidal forms. Single tuning control
-Sound powered headphones
-Antenna, 40 meter wire up 3 - 4 meters running east to west.
Robert Weaver, Active Device Class
This year, I have two different active device receiver entries. One is a direct conversion receiver using a sheet beam tube, that I built two years ago, and used in the Radioboard summer DX contest in 2008, but never in this contest. Shortly after I built it, I built a sheet beam superhet receiver and the direct conversion receiver was mostly forgotten. I wanted to dust it off, make a few improvements and enter it in another contest. It took a while to get to it. So, I began the contest using the same superhet that I entered last year. That receiver is completely unchanged, and you can find a description of it under the 2009 entries, or see the writeup on my webpage.
The direct conversion receiver has a tuned front end to help reduce interference from strong local stations and out of band signals. The signal enters the control grid and is mixed with the oscillator signal which appears on the deflectors. The result is an audio signal which appears as a differential signal on the plates. This audio signal drives the push pull primary of the interstage audio transformer. The secondary is connected back to the control grid in series with the incoming RF so that it is amplified reflex style. The resulting amplified audio is now a common mode (in-phase signal) at the plates, and so has no effect on the interstage transformer, and passes through the primary winding and on to the output transformer which is in series. The secondary of the output transformer drives the headphones.
Feedback for the local oscillator is taken differentially from the plates, through an audio blocking capacitor to the primary of the oscillator coil. One half of the secondary of the oscillator coil forms a tuned circuit with one gang of the variable capacitor. This side of the secondary drives one deflector, and the other half of the secondary produces an opposite phase signal to drive the other deflector. This oscillator is very stable and is almost completely immune to pulling towards the incoming RF. In fact this is a bit of a disadvantage, because it makes it very touchy tuning in AM signals without getting a beat frequency.
In the first version of this receiver, I had included feedback from the cathode to a one turn tickler on the RF coil, to provide a small amount of regeneration. However, after making a number of modifications and tests, I finally concluded that there was little to no benefit. So, I finally removed it. A description of the receiver is here.
There is a DPDT switch which reverses the phase of the interstage transformer secondary. This is to correct for slight unbalances in the tube which could cause oscillation when the volume control is turned all the way up. There is a bit of phase shift that varies across the band, and so the best position of the switch changes at different frequencies. I had tried various other plate balancing techniques (such as in my superhet receiver) but found that nothing worked as well as the reversing switch.
The receiver worked reasonably well. It was built as an experiment to see what could be done with a sheet beam tube. If you read the write up on my website, I sounded much more enthusiastic right after I built it. However, nearly two years later, and having built a better receiver in the interim, I find that listening to AM for any length of time on this set can be tiresome with the touchy tuning and almost constant beat tone. So, after using it for a couple of days and logging a respectable number of stations, I retired it and switched back to the superhet which outperforms it in almost every respect.
It seems that each year, I manage to learn a new trick or two. This year, I connected a set of wavetraps between my loop antenna pickup coil and the receiver. I wasn't sure how wall wavetraps would work with the loop. But, using the antenna orientation to null out a local station on 860kHz, and then using two of the wavetraps tuned to the same frequency to further null it out, I was able to log my first station ever on 870 kHz: WWL New Orleans, which by the distance/power formula was my second best catch of the contest. My best catch was KVNS 880, Brownsville TX. Attempts to log a MW station outside of North America have failed so far. Hopefully, one of these days it will happen.