Litz wire for AM ferrite Rod Antenna?
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From: YD on 28 Mar 2007 18:53 Late at night, by candle light, "Bill Bowden" <wrongaddress(a)att.net> penned this immortal opus: <schnipp-schnapp> >On Mar 27, 8:01 am, YD <ydtech...(a)techie.com> wrote: >> >> Ack, I have a 200 kW 600 kHz transmitter just over a kilometer away. >> Great for RFI immunity testing. >> > >Is that KOGO clear channel (600Khz) in San Diego? >Up here in LA area, we have clear channel KFI on 640KHz. >They both carry Coast to Coast with Art Bell, George Noory, >etc. Some of the stuff is pretty good, but I stay away from the remote >viewing, flying saucers, "out of body experiences" and shadow people >stuff. Some of the callers ghost stories are very original. I like >good story tellers, even if it's BS. > >-Bill > Nope, it's Radio Gaucha in southern Brazil. When they installed it about 15 years back you could hear it on the telephone! Great for soccer fans but a pain for everybody else. Then they had some crews running around placing filters and what-not all over town and it's pretty much under control now. It still gets in if you're not careful and crystal radios are a waste of time. - YD. -- Remove HAT if replying by mail.
From: Rich Grise on 28 Mar 2007 19:07 On Tue, 27 Mar 2007 15:33:53 -0700, John Larkin wrote: > On Tue, 27 Mar 2007 13:01:12 -0300, YD <ydtechHAT(a)techie.com> wrote: >>Late at night, by candle light, John Larkin >>>On Mon, 26 Mar 2007 22:27:55 GMT, Rich Grise <rich(a)example.net> wrote: >>>>On Sun, 25 Mar 2007 22:40:34 -0700, Bill Bowden wrote: >>>>> On Mar 25, 5:35 pm, John Larkin >>>>>> On 24 Mar 2007 18:53:56 -0700, "Bill Bowden" <wrongaddr...(a)att.net> >>>>>> >On Mar 24, 8:22 am, John Popelish <jpopel...(a)rica.net> wrote: >>>>>> >> Bill Bowden wrote: >>>>>> >> > How much improvement can I expect using Litz wire to wind a AM ferrite >>>>>> >> > Rod antenna as opposed to using solid copper wire? >>>>>> >>>>>> >> > Is it worth the trouble to obtain Litz wire, or can I expect almost >>>>>> >> > the same response at say 1 MHz using regular solid enamaled copper >>>>>> >> > wire? >>>>>> >>>>>> >> If you would like to see some comparative experimental data, >>>>>> >> Ben Tongue has performed some experiments and posted the >>>>>> >> data to his web site.http://www.bentongue.com/xtalset/29MxQFL/29MxQFL.html >>>>>> >>>>>> >Yes, it looks like Litz wire has a significant advantage. If I read >>>>>> >the results right, the unloaded Q factor at 943Khz is 141 using solid >>>>>> >copper wire verses a Q factor of 1030 using Litz wire. >>>>>> >That's quite a significant difference. Am I reading the results right? >>>>>> >>>>>> In a superhet, high Q will make it that much harder to track the LO, >>>>>> so you may well lose signal with a q=1000 rod. Why do you want a high >>>>>> antenna Q? In the AM band, gain is cheap and s/n is dominated by >>>>>> ambient noise, so it won't matter much. >>>>> >>>>> It's just a little portable AM radio I've been wanting to build for >>>>> years. I took a radio class in 7th grade 50 years ago and never got >>>>> around to finishing the superhet design. But I got an A anyway. We >>>>> used tubes in those days. >>>>> >>>>> I'm using the Signetics NE602 balanced modulator IC that produces >>>>> about 13dB gain. The antenna rod is buffered with a JFET so there is >>>>> minimal load on the antenna rod yielding another 12 to 18 dB. The >>>>> front end is pretty hot. >>>>> >>>>> But as you say, the bandwidth is narrow with a high Q coil, so I'm >>>>> using a switch to short a couple turns on the antenna rod to load the >>>>> antenna for local strong stations. Local/DX select. >>>>> >>>>> The biggest problem is separating a distant station 40KHz away from a >>>>> strong local 50KW station 5 miles up the road. >>>> >>>>I've read somewhere that the idea is to build the selectivity into the >>>>IF part. >>> >>>Yes, but you have to be careful that nothing overloads in the >>>front-end, or in the first IF stage, from that 50 KW monster. A decent >>>antanna Q helps some. Fets are really good for avoiding nonlinearity. >>> >>>So, low-gain jfets or mosfets in the front end and maybe the first IF, >>>and pile up selectivity and gain in later IF stages. Manual stage gain >>>pots, rather than AGC, would be fun. >> >>Ack, I have a 200 kW 600 kHz transmitter just over a kilometer away. >>Great for RFI immunity testing. > > We look up at this beast from our back window. Something like 22 > megawatts of AM, FM, TV, and HDTV... > > http://en.wikipedia.org/wiki/Sutro_Tower > > and we're in a wooden building... no shielding at all. All the scope > traces are fuzzy. > Doesn't it have a pattern something like this? ("side" view): http://www.marcspages.co.uk/tech/antgain2.gif IOW, aren't you kind of "in the shadow"? Cheers! Rich
From: Bill Bowden on 2 Apr 2007 16:05 On Mar 24, 7:16 pm, John Popelish <jpopel...(a)rica.net> wrote: > Bill Bowden wrote: > > On Mar 24, 8:22 am, John Popelish <jpopel...(a)rica.net> wrote: > >> Bill Bowden wrote: > >>> How much improvement can I expect using Litz wire to wind a AM ferrite > >>> Rod antenna as opposed to using solid copper wire? > >>> Is it worth the trouble to obtain Litz wire, or can I expect almost > >>> the same response at say 1 MHz using regular solid enamaled copper > >>> wire? > >> If you would like to see some comparative experimental data, > >> Ben Tongue has performed some experiments and posted the > >> data to his web site.http://www.bentongue.com/xtalset/29MxQFL/29MxQFL.html > > > Thanks John, > > > Yes, it looks like Litz wire has a significant advantage. If I read > > the results right, the unloaded Q factor at 943Khz is 141 using solid > > copper wire verses a Q factor of 1030 using Litz wire. > > That's quite a significant difference. Am I reading the results right? > > I think you are interpreting this correctly.  Keep in mind > that this is the Q of the coil, unloaded by any receiver > circuit.  If the circuit adds a significant load, the Q > differences for a tuned antenna would be a smaller ratio > different.  22 AWG is also pretty heavy wire for a typical > antenna coil.  With smaller wire, the Qs would be smaller > but closer. > I measured the Qs of a couple ferrite rods, one with litz wire and the other with solid wire of about the same AWG. Both Qs measure around 80 unloaded. I don't see any difference using litz wire. I used a single loose turn of wire to couple a generator to the LC circuit at 533KHz. The 3dB rolloff points occur at around 529KHz and 536KHz for a bandwith of about 7 Khz and Q of 533/7 = 76. The rod with the litz wire is loaded by the gate of a jfet, so I believe there is no load, the other rod had no external loads connected. Not very impressive results, but the bandwidth seems reasonable, and the audio quality is good using a little 2.5 inch speaker. -Bill > > > > Quote from website: > > > " Solid wire instead of litz?:  Keep in mind that the work described > > here used close-wound 125/46 litz wire.  If one duplicates 'Coil and > > Former B' in Table 2, except using 22 ga. solid copper wire (having > > the same diameter) as 125/46 litz, the Q values drop to about 1/6 of > > the values achieved with the litz wire.  The cause is the large > > proximity effect resistive losses in the solid wire.  The proximity > > effect, but not the skin effect loss may be much reduced if the wires > > are space-wound.  New trade-offs now must be considered: Same wire > > diameter, and therefore a longer solenoid, or a smaller wire diameter > > and the same overall length?  If one wishes to use solid wire, it > > should probably be wound directly on the ferrite, not on a former. > > The overall Q will still be much less than when using litz, but the > > loss from the high (tan δ) dielectric of the ferrite will be pretty > > well swamped out because of the now higher losses from the skin and > > proximity effect losses.  The Q values, using a close-wound solenoid > > of 22 ga. solid copper wire on a polyethylene former, as in 'Coil and > > Former' B in Table 2 are: 520 kHz: 130, 943 kHz: 141 and 1710 kHz: 150 > > when using the "best core".  The Q drops only 3, 3, and 5 points > > respectively if the "worst core" is used. " > > > -Bill- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
From: John Popelish on 2 Apr 2007 15:55 Bill Bowden wrote: > I measured the Qs of a couple ferrite rods, one with litz wire and the > other with solid wire of about the same AWG. Both Qs measure around 80 > unloaded. I don't see any difference using litz wire. > I used a single loose turn of wire to couple a generator to the LC > circuit at 533KHz. The 3dB rolloff points occur at around 529KHz and > 536KHz for a bandwith of about 7 Khz and Q of 533/7 = 76. > > The rod with the litz wire is loaded by the gate of a jfet, so I > believe there is no load, the other rod had no external loads > connected. > > Not very impressive results, but the bandwidth seems reasonable, and > the audio quality is good using a little 2.5 inch speaker. Sounds pretty reasonable to me. With that Q, I suspect the limit is more the ferrite than the wire type. How much of the rod's length is covered with the coil? Turns in the middle third produce higher Q than turns at the ends. Is the wire wrapped right on the rod, or over some sort of insulating tube? Spacing the wire about a wire diameter or so from the rod can improve the Q at the upper end of the band.
From: Bill Bowden on 2 Apr 2007 23:10
On Apr 2, 12:55 pm, John Popelish <jpopel...(a)rica.net> wrote: > Bill Bowden wrote: > > I measured the Qs of a couple ferrite rods, one with litz wire and the > > other with solid wire of about the same AWG. Both Qs measure around 80 > > unloaded. I don't see any difference using litz wire. > > I used a single loose turn of wire to couple a generator to the LC > > circuit at 533KHz. The 3dB rolloff points occur at around 529KHz and > > 536KHz for a bandwith of about 7 Khz and Q of 533/7 = 76. > > > The rod with the litz wire is loaded by the gate of a jfet, so I > > believe there is no load, the other rod had no external loads > > connected. > > > Not very impressive results, but the bandwidth seems reasonable, and > > the audio quality is good using a little 2.5 inch speaker. > > Sounds pretty reasonable to me. With that Q, I suspect the > limit is more the ferrite than the wire type. > > How much of the rod's length is covered with the coil? > Turns in the middle third produce higher Q than turns at the > ends. > > Is the wire wrapped right on the rod, or over some sort of > insulating tube? Spacing the wire about a wire diameter or > so from the rod can improve the Q at the upper end of the band. The rod with litz wire is a original unit from a 1972 Radio Shack AM/ FM modle. The rod is 4.75 inch long by 3/8 diameter with a winding length of about 1 inch (on a paper form) situated about 1/3 the way from one end to allow fine tuning adjustment by sliding the winding slightly up an down the rod. The other rod with solid wire, I wound myself. It's a little shorter at 4 inches (same diameter) and the winding is similar 1 inch long situated in the middle of the rod on a similar paper form. I'll run another test at the upper end of the band to see what happens there and report results. I etched the circuit board with a solid ground plane on one side to avoid possible oscillation problems, but I found I need to space the antenna rod about 1/2 inch away or more to avoid losing signal. This makes the box size bigger than I planned, but I can live with it. Guess it's not a good idea to use a solid ground plane with little AM radios? -Bill |