Re: [loopantennas] Plastics usage and the Pollydoroff 4 coil STLS antenna
- Well Guys,PVC would not be my first choice, however, it was the only thing that I could come up with that would work for my project. I know that Ben Tongue and the guys on Rap and Tap do not like PVC, but as Tod has mentioned those are the Crystal Radio gurus, where selectivity is very important.All of that being said, Stormwise, whom sell their Monster Ferrite Rods, which are Ferrite slugs that are epoxied inside a thick walled PVC pipe and than there are the Brazilians whom have a lot of info on the web with their bundled Russian ferrite rods, which are housed inside of PVC pipe. So others are using it, even though it is not the best choice of materials.I have no way to measure Q, however, I can say that the tuning is very sharp, which is an indication of high Q. I also saw very good rotational nulls, much better than my 1 meter air loop. What I did notice is that the tuning range was reduced from 400-2290 KHz to 400-2244 KHz, when I placed the 2" PVC pipe over the finished coil, so it did add a very minor amount of capacitance to the circuit.I like the 660/46 Litz wire, even though it may have been over kill for this project, because it is easy to use, other than the soldering. It has about the same cross section area as 18 gage wire, but it is not springy, so the wire raps stay in place, it is like using a piece of string.Based on my may experiments with different winding configurations I have found the Pollydoroff design, that I used in this project, to be a very good performer, over other configurations. However, in some of my earlier experiments I got some very strange results, like very wide tuning and sharp nulls on the high side of the station, but no nulls on the low side of the station. What I later found out was that you need a lot of space between the coils. I have had very good results with 1 1/2 to 3" of space between the coils. At some point I am going to try even wider spacing and I may try 6 coils. I have enough of the toroids to build a wider rod. The rule of thumb for the L/D ratio is 10:1 to 20:1, so I could go up to 28" on rod length, which should work well for a 6 coil Pollydoroff design loop.EverettIn a message dated 2/24/2013 8:59:28 P.M. Central Standard Time, toddroberts2001@... writes:
Most crystal radio set builders have settled on Polystyrene
as the best material for highest-Q coil forms. Some crystal set
builders have built coils with Qs up to 2000 and above.
Once you get a good coil form material then the type of
wire used makes the biggest difference in Q with Litz Wire being
However receiving loop builders don't necessarily want the
highest Q as it will narrow the bandwidth too much making
the tuning very narrow and peaky and the audio sound narrow and muffled.
Some builders purposely use plain enameled wire instead
of Litz Wire for this reason. It will need to be trial and error
to find the best all-around Q without being too narrow.
But if you are going for all-out Q then try to use Polystyrene
for the coil form or supports and heavy Litz Wire like #660/46 for
broadcast band use.
73 - Todd WD4NGG