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|gzimmer||Posted - 16 October 2012 20:14 |
It's sufficient to just connect the dipole feeders to the Ant/Earth terminals. Not strictly symmetrical, but that doesn't seem to matter much.
I tried two ATUs and a change-over switch to select between balanced and un-balanced, but found it not necessary. The two configurations tune much the same, just a slight tweek of the ATU tuning to go from one to the other.
Edited by - gzimmer on 10/16/2012 8:15:33 PM
|Steve McDonald||Posted - 16 October 2012 23:7 |
OK Zim...so basically it is acting more like an inverted 'L' or a short top-loaded end-fed vertical wire with the unfed other half of the antenna contributing something different than what you might expect to see in a normal dipole...or are you able to tunr that leg as well?...just trying to picture what you have there. What happens to a signal(s) when the grounded feeder is disconnected from the detector input / ground ...any change? Like you, I often thought it would be great to be able to feed my 160m coaxial-fed (50 ohm) half-sloper into the front end of my DX set (when I had it operational a few years ago) but never played with something that would match it...still intrigues me though :-)
WEB - "The VE7SL Radio Notebook":
|gzimmer||Posted - 16 October 2012 23:23 |
The "earth" terminal on the set isn't connected to earth....
The dipole and feeder are balanced, and while the feeder should connect to a balanced link into the radio, simply connecting it to the existing antenna and earth terminals is sufficiently balanced in my case (without an actual earth connection of course).
In my case the ATU is itself link coupled to the next stage, so it is sufficiently isolated from ground to call it a balanced input. The only asymmetry is the single cap on one side. I did try a completely symmetrical input but it wasn't worth the extra complication.
To clarify further, I have four possible configurations:
I was surprised that even the last two combinations give decidedly different signal pick-up.
|Steve McDonald||Posted - 17 October 2012 13:30 |
OK...I have it now Zim. I gather that the original configuration of the feedline, before splitting it into two wider-spaced lines would have increased the feedline impedance substantially and provided a better match into your front end tuner??
Is there a clear 'winner' amongst the four combinations? (I'm guessing #3 if any).
I recall playing with the coaxial fed 160m sloper on the top end of the BCB with an old tube radio several yeras ago and sitting on one of the graveyard channels, listening to the mix of signals. Lifting the shield from the coax would often pop one signal right out of the mess. I guess the thing was acting more as a vertical wire at that point rather than a horizontal wire with some associated directivity.
|_J_||Posted - 18 October 2012 10:25 |
I haven't calculated anything but it seems these lead-ins are not functioning much like matched or tuned T-lines. Here is my simple minded view of these:
1. Dipole, should work well for skywaves if half wave long and far enough from ground. If not, (more realistic) its difficult to predict since the feedline affects it so much.
2. Virturally a “T” since if short compared to a half wave, and has similar currents in each side of lead in (regardless: currents in each side of feeder are forced = at connection and forced ~= by C coupling everywhere else). Should work well.
3. and 4. Inverted “L” variations, each capacitively coupled to its mirror inverted “L” in opposite direction. Complex to predict but with ordinary (coax, twinlead) feedlines the cap coupling may make them operate much like #2. Wide latterline may offer a different result.
Edited by - _J_ on 10/18/2012 10:28:53 AM
|gzimmer||Posted - 18 October 2012 10:56 |
John, I suspect that your definition of Tuned Lines is different to mine. To me Flat lines are those terminated in their characteristic Impedance. Tuned Lines however are those which are purposefully operating with large mismatches (e.g. high SWR) and are tuned at the bottom by an ATU.
> it seems these lead-ins are not functioning much like matched or tuned T-lines.
In the dipole version, the length of T-line transforms the antenna impedance somewhat, and then that new impedance is absorbed/cancelled by the conjunctive match at the ATU. The antenna is both brought to resonance and impedance matched by the ATU via the Tuned-Lines (assuming the ATU is correctly adjusted).
The feed-line can have no effect on a dipole as long as it is balanced and at right angles. In that situation it is invisible to the antenna.
Edited by - gzimmer on 10/18/2012 12:22:15 PM
|gzimmer||Posted - 18 October 2012 11:4 |
Edited by - gzimmer on 10/18/2012 12:20:12 PM
|_J_||Posted - 18 October 2012 16:43 |
I think the feedline will affect the dipole and not be transparent if the dipole is not a half wave (which I assume is the case) and if the dipole is not far enough away from ground (assume is the case too). So thats why I expect both of these to make the antenna drive point Z unlikely to be equal to the feedline. Also the feedline would likely be short and transform the Z to something that would take calcs to predict.
|gzimmer||Posted - 18 October 2012 20:2 |
I still don't understand what you are trying to say:
> So thats why I expect both of these to make the antenna drive point Z unlikely to be equal to the feedline.
The feed-line impedance is unknown and the antenna will only be resonant at one frequency, so yes, the feed-line is mis-matched and will have SWR.
> Also the feedline would likely be short and transform the Z to something that would take calcs to predict.
The feed-line will have SWR and will transform the termination impedance to something new. This will happen whatever the length, short or long.
So how will the feed-line affect the antenna (other than bringing it to resonance as intended) ?
|giftsocool||Posted - 18 October 2012 20:48 |
If I use a fully resonant, MW, A/G circuit eg. Variable ferrite coil resonated against <=100ft antenna capacitance, the impedance across the band will be ~Rg?
I have same duobt PLZ HELP
|gzimmer||Posted - 18 October 2012 21:15 |
If you read the thread you will see this has already been answered.
If you tune out the reactance with a series inductor (or whatever), all that is left is the resistance of the antenna plus ground.
This resistance will vary from very low (for a short antenna) to 75 ohms or so (for 1/4 wave length) to very high (for a 1/2 wave length).
As before, the graph at www.qsl.net/vk3gjz/Misc/ZimAntGraph.JPG shows what happens for a 100 metre wire.
On the right axis (and in blue) you can see how the reactance changes. On the left axis (and in red) you can see how the resistance changes.
You can see how the reactance goes to zero (eg the antenna becomes self resonant) at both the 1/4 and 1/2 wave points.
For shorter antennas you can just use the left-most portion of the graph.
|_J_||Posted - 18 October 2012 21:23 |
OK, Zim. When you said "The feed-line can have no effect on a dipole as long as it is balanced and at right angles. In that situation it is invisible to the antenna." I think you assumed feedline matched (72 ohm) to antenna feedpoint. But for a short dipole close to ground the feedpoint would not be 72 ohms or to the Z of any ordinary T-line. Mismatching an antenna affects it. What am I missing?
Edited by - _J_ on 10/18/2012 9:25:41 PM
|gzimmer||Posted - 18 October 2012 21:28 |
> What am I missing?
The simple fact that this feed-line is NOT matched to the antenna. It is purposefully operated in a mis-matched condition (as are all tuned lines).
Consider the special case where a feed-line stub is used to match an antenna (eg a G5RV antenna). By selecting the impedance and the length of the stub (via Smith chart) you can transform the reactance of the antenna to whatever impedance you wish (eg 50 ohms or whatever).
In this case however no attempt is made to use the feed-line itself to match the antenna. The feed-line simply transports the antenna impedance (altering it along the way) down to the ATU. The ATU then does the tuning and loading.
Edited by - gzimmer on 10/18/2012 9:52:30 PM
|_J_||Posted - 19 October 2012 9:40 |
In that case, seems like the feedline affects the antenna, by tuning it to frequency?
|gzimmer||Posted - 19 October 2012 9:54 |
> In that case, seems like the feedline affects the antenna, by tuning it to frequency?
Yes, exactly. This was my point from the beginning.
So what on earth was the point you have been trying to make?
Edited by - gzimmer on 10/19/2012 10:07:36 AM
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