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|golfguru||Posted - 21 August 2012 18:46 |
Serious question :-
Is there an absolute max/min impedance range that one could expect to see when looking out at an untuned, MW, longwire, A/G system. (eg. Zero, with perfect ground, up to a few thousand ohms, at "whatever" freq.)?
Would length a factor?
Edited by - golfguru on 8/21/2012 6:47:36 PM
|gzimmer||Posted - 21 August 2012 22:55 |
It depends on Length and Frequency.
A quarter-wave (with a good earth) will be around 50 ohms, while a half-wave will likely be over 5K ohms.
Non-resonant lengths however will have a complex impedance. Shorter than a 1/4 wave will be Capacitive in series with a low Resistance.
For short antennas, you could say that the complex impedance is "high", however if you tune out the Capacitance with added Inductance, the remaining resistance is rather low.
That graph I posted gives a good picture. Just pretend that bottom axis is Length instead of Frequency.
|golfguru||Posted - 21 August 2012 23:1 |
|_J_||Posted - 23 August 2012 9:50 |
Looking at this another way, the short answer is no. You said "untuned" but there essentially is no "untuned" antenna. Every antenna is tuned to something.
You can change the frequency that the antenna (and associated tuner function) is tuned to by a process called 'tuning' but to omit that process and/or function doesn't really mean the antenna is 'untuned'. It just means the antenna is incorrectly tuned; so the issue becomes "how incorrect" is the tuning.
It depends on how far off its 'tune' you are using the antenna, but in theory the range is essentially infinite.
Does that make any sense?
Edited by - _J_ on 8/23/2012 1:41:38 PM
|golfguru||Posted - 23 August 2012 17:18 |
Was wondering if a switchable balun could better match to the antenna across the MW band when used in an "untuned" situation.
|Garry Nichols||Posted - 23 August 2012 18:15 |
Baluns usually work best in a matched situation. That is, the load is pure resistance or close to it, meaning the antenna is close to resonance.
(What you would want for an unbalanced load would be an "unun". It will transform X unbalanced resistance to Y unbalanced. )
For your situation, all you have to do is hook up a series or parallel L/C to the antenna and ground, resonate it to the MW freq you wish to listen to and then couple it to the crystal set in some way.
I don't think that there is any way to make a too short, very low R antenna look like something better other than a matching unit, which is what the L/C would do for you.
If the antenna is always below quarter wave resonance, you could get away with just a coil and a tap. Just move the tap along as you tune the band. But I've always found that tracking the set goes much better with a VC in the antenna circuit.
You will wind up with the good ole double tuned set.
Edited by - Garry Nichols on 8/23/2012 6:17:33 PM
|golfguru||Posted - 23 August 2012 18:33 |
>>>> You will wind up with the good ole double tuned set.
Yes. That's what I was trying to avoid. ie. I am trying to maximise the performance of a single tuned set.
|Richard||Posted - 23 August 2012 22:2 |
"I am trying to maximise the performance of a single tuned set."
A silk purse from a sow's ear, huh? Good luck, Golf. Didn't the alchemists of old attempt something similar with Pb into Au? ;)
Edited by - Richard on 8/23/2012 10:08:57 PM
|golfguru||Posted - 23 August 2012 22:5 |
The last big hurdle to get over :-)
|gzimmer||Posted - 24 August 2012 3:28 |
My long wire is a 1/4 wave near the bottom of the band and a 1/2 wave near the top, so its impedance changes dramatically across the band.
The little wide-band transformer I've been talking about gives a good match right across the band (with different taps), particularly at the resistive points (1/4 and 1/2 wave). Adding a simple tunable L/C in series with the antenna improves results away from the resonant points (and adds useful selectivity everywhere).
The really curious thing is how the wide-band transformer alone can force the antenna into it's various resonant modes.
I have two local stations which are near the 1/4 and 1/2 wave frequencies. With a diode on the top of the transformer, I can switch between the two stations by altering the taps. On a low-Z tap the antenna is forced into 1/4 wave mode and I can hear one station, then on a high-Z tap the antenna is in 1/2 wave mode and I hear the other station. The only selectivity is that of the antenna itself.
|John Bruce McCreath||Posted - 24 August 2012 8:59 |
I'm interested in your wide-band transformer. I'd appreciate some details of it.
73, J.B., VE3EAR
|Garry Nichols||Posted - 24 August 2012 11:6 |
You appear to be canceling out what I explained to golf' ! (I do not have a problem with that ! )
I'm not sure why you are trying to put some sort of circuit between the antenna/ground and your single tuned set. Connected directly to A/G it will tune the wires across the band if the tank and VC are adjusted for the range.
Tapping down the tank coil will aid in matching the A/G.
I've tried this and it works fine (as Zim has said many times!). My problem is SW interference. It is often difficult to hear MW stations because of SW.
With my understanding, a simple "ultimate" single tuned setup is a parallel L/C, antenna tap varying on the L, and diode tap varying on the L. I think you get to match the antenna and the diode fairly well with that setup as you tune the band.
Al has a method of reducing SW, but you need to go with an untuned antenna coil:
I'm going to try this set one of these days. In recent correspondence he told me that I was the first one to respond to his URL on this idea.
|_J_||Posted - 24 August 2012 13:1 |
Here's my view.
The basic function of the tuner in a single tuned set is to match BOTH the imaginary (tune the antenna to the freq) and real (resistance match) of the antenna to the set.
I think there may be a lot of misunderstanding about the need to add a matching circuit or tuner to a radio. Adding that to a single tuned set essentially makes it a dual tuned set, and best designed as an integrated part of the set. Done right, that can increase the Q a lot, but is not needed just to basically tune and match the set.
The single tuned set, done properly, tunes and matches the antenna COMPLETELY, provided there are 2 "degrees of freedom," (which boils down to 2 controls), 1 to adj frequency and one to adj loading (for example a single coil with 2 sliders). They usually interact, but oh well... Really simple, nothing more is needed.
Edited by - _J_ on 8/24/2012 2:47:18 PM
|gzimmer||Posted - 25 August 2012 5:45 |
This is something I've often thought about....
With a basic (single coil) set, the antenna can be tapped down the coil (or down the Cap) to provide the required match.
As John says, you need two adjustments: The Tap position provides the impedance matching, while the Tuning Cap provides the tuning. If you like, the antenna "steals" the required inductance from the tuned circuit. Or to put it another way, the tuning position is different with the antenna connected (from what it would be without the antenna). Of course the two adjustments strongly interact, which can make tuning rather difficult (eg the split Tuggle).
Now the above assumes that the Antenna and Tuned Circuit are tightly coupled. However we usually trade Sensitivity for Selectivity, and so we adjust the coupling on the light side of optimum. I suspect that when the two are only lightly coupled the Antenna cannot "steal" the Inductance it needs.
The antenna itself acts as a tuned circuit (if it is resonant) so the optimum coupling (and Selectivity) occurs when the antenna and the Tuned circuit are both tuned and only lightly coupled. I don't think this can happen if the antenna is "stealing" the required Inductance from the main tuned circuit.
So instead, it is perhaps better to provide the required Antenna Inductance separately by putting a variometer (or a coil and var cap) in series with the Antenna. Then the Antenna Tuning and Loading adjustments are separate and don't (much) interact. Of course you now have a third control (main tuning) but the tap position now acts a simple coupling (Selectivity) adjustment.
But now that we have another coil, we start to think how we can raise its Q and gain some Selectivity. This then brings us back to our starting point (we are going around in a circle).
So I conclude that because the Q of the Tuning coil is decided by the antenna Resistance, it is best left alone to do only the one job. It still contributes some useful Selectivity though.
My favorite circuit then is to have a variometer (or equivalent) in series with the antenna, and then tap it down the main coil. There are dozens of possible variations on this scheme of course.
All this makes my brain hurt....
later ........ Zim
Edited by - gzimmer on 8/25/2012 6:24:19 AM
|gzimmer||Posted - 25 August 2012 6:18 |
The little broad-band transformer has been discussed a few times here, but I'm traveling at present so I'm not sure if I can find the details...
It uses a ferrite toroid (Jaycar #LO1238). The particular ferrite has very high AL, giving high inductance per turn.
The primary has 120 turns, tapped every ten turns. The Secondary (if needed) is one or more turns as required. Normally however it used as a simple auto-transformer (no secondary).
It was based on the CoilCraft series of wideband transformers
My one seems to be better than the Coilcraft version and has the advantage of multiple tapping points.
I might be able to find my original rough drawing, or perhaps someone else has copy.
later .......... Zim .......... VK3GJZ
Edited by - gzimmer on 8/25/2012 6:52:17 AM
|John Bruce McCreath||Posted - 25 August 2012 7:23 |
Thanks for the details on your transformer. I built an unun a while ago to better match a random wire aerial to my portable shortwave receiver. It has 4:1, 9:1 and 16:1 ratios depending on which tap I use of the 40 total turns. The core was one I had in my junk box of unknown origin, but the total L of the unun is 1200 uH.
73, J.B., VE3EAR
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