Steve's All Band Unicoil
OK I think it is important to highlight some important details and the history behind the idea for this Unicoil. It originated as a need for new coils in my Lafayette radio retrofit. The four coils that were in there just didn't have the performance I was looking for. Small wax and paper coils just don't have enough Q to render respectable performance. There may be exceptions but I am sure they are few and far between. I just had to find a way to replace those junky coils and fit them into the small allotted space under the chassis of my Lafayette.
While working on solutions to other issues on that radio at the time I happened to run across a radio project featuring the 12AL8. It was a two tube radio built by a person who's call sign is K5BCQ. While certain features of the radio were interesting it seemed a little clunky. Four stages and still only headphone output? Maybe due to the triode detector issue I addressed with my radio. I did notice something very unique and slightly puzzling about it, the coil. It was capable of receiving on 3 different shortwave bands with the same coil. After that I thought about it wondering if I could make something like that work for all bands. Well I did it and took it to a whole new level! There are notable differences separating my idea from his, these will be detailed below. I decided to coin it the "Unicoil".
The original prototype is wound on a 2" PVC pipe form. See the pictures I sent for a visual and further explanation. The original did not have any spacing between the taps, but I soon figured it would be an improvement if it did. The coil did work but needed improvement and resizing. It would still work well on a BIG SET. The second coil wound on 1 1/4 PVC has space between the finer windings and on the larger winding the insulation of the wire acts like space. With magnet wire I highly recommend .050 spacing between the taps and .150 between the last winding and the tickler. The new one I am making will have space between all of the windings and will have a greater variation of wire gauges amongst them. It too will be done on 1 1/4 PVC.
This is indeed not just a coil with random taps. The unused windings a shunted out, electrically isolating them from the rest of the coil. I used a six position rotary switch with a bridge jumper. The spacing further isolates them with no degradation in performance. I have tested coil two next to my high Q Ocean Hopper coils and the performance is the same across the board! That was pretty exciting news.
After I have achieved the desired coverage and other details I will use these coils in more sets. They can really save time and space. They don't occupy any more space than a good standard broadcast regenny coil!
I didn't want to bore you with a long winded explanation, but I think this idea was worth it. In a few days I will update you with the details of the new coil that will be installed and running.
I have some numbers to run by you but it is getting late. By the way the page looks good, thank you. That was my first prototype coil and is not as good as my current design. I just wanted to note that. As I get the latest updates I will keep you informed.
Here are the numbers:
Band A: 222.5 uH @ 50-550 pf .45 - 1.5 mhz 50 turns 1.89" total space 32ga Band B: 58.9 uH @ 50-550 pf .88 - 2.9 mhz 20 turns 1.44" total space 27ga Band C: 22.6 uH @ 40-180 pf 2.4 - 5.3 mhz 10 turns .95" total space 24ga Band D: 9.2 uH @ 40-180 pf 3.9 - 8.3 mhz 7 turns .68" total space 24ga Band E: 2.7 uH @ 40-180 pf 7.2 - 15.3 mhz 4 turns .48" total space 20ga(space wound) Band F: 0.6 uH @ 40-180 pf 15.3 - 32.5 mhz 3 turns .19" total space 20ga(space wound)
Coil gets wound on a 1 1/4 inch PVC pipe 1.625 OD. All coils including spaces occupy 2.33 inches of length. These are all calculated numbers and are subject to change upon making the real thing. I would recommend the total length of the pipe itself to be at around 3" for manageability.
All windings to be done with the specified guages of magnet wire. .050" spacing between each winding and .150 space between the last and the tickler. The tickler itself will have a total of 13 windings 24ga tapped at 3,4,6 from the main going out.
Please note the use of a 30 pf bandspread trimmer will give additional overlap in coverage and benefit the higher ranges greatly.
I am a quite tired so I will proofread these specs tomorrow, but I believe I said everything correctly. I will check back tomorrow. Thanks,
I just wanted to check back and ask see if you checked out the coil stuff. The variable capacitor used for this setup is a common 370/180 variable found in 7 out of 10 "All American Five" tube radios making it easy to obtain for prospective builders. There can be leeway in the values 20pf or so in either direction, that could be absorbed with an optional 50pf or so band spread control anyway. I like to use band set and band spread controls in most of my better projects. In the first two bands both section of the cap are used, hence 550pf. I the subsequent four bands the smaller 180pf section is used. Switching can be accomplished either through a multi section rotary switch ganged with the band selector or a separate toggle. It is entirely up to choice or convenience. I just wanted to mention that in case you were wondering about the funky capacitance values and such.