The Xtal Set Society
The Xtal Set Society
profile | register | search | main site


email us
Forums | Fireside Crystal Radio Clubhouse | Dipole antenna Post Reply Send Topic To a Friend
Author Topic
_J_Posted - 13 October 2012 18:49  Show Profile  Email Poster  Edit Message
Well, maybe, but more precisely, it will be exactly the combination of Rg, Rr, (and other losses if they are significant) but it will be approx Rg if Rg is the 'tall pole in the tent' which happens if the antenna is too short or if the ground resistance is high or both.

If you have a really good antenna and a really good ground, it should be closer to Rr.

Make sense?

John Davidson

_J_Posted - 13 October 2012 19:5  Show Profile  Email Poster  Edit Message
GG,
I had a 110' long wire up at about a 45" angle (up a hill to a really tall palm tree) and a couple of #12 wires buried under it and 3 pipes about 5' in the ground at the feedpoint, pretty good. My Rr was about 7 ohms (this included some losses besides Rr, but...) and Rg was about 20 ohms (this is at RF, not DC, and included some other dirt losses), so my drive point resistance was about 35 ohms (at RF).

Bare in mind this needs to be measured at RF because DC measurements will lie to 'ya.

(corrected the Rr!)

John Davidson

Edited by - _J_ on 10/13/2012 8:16:16 PM

golfguruPosted - 13 October 2012 19:45  Show Profile  Email Poster  Edit Message  
So because of (radiation?) resistance it won't be consistant across the band?

Or close enough not to matter? (on a <1/4 wave antenna).

Or does Rr not have any "unreal" bits?
ie. Is (Rr+Rg) a constant across the MW band.

Thanks.

..................

Edited by - golfguru on 10/13/2012 7:50:09 PM

_J_Posted - 13 October 2012 20:18  Show Profile  Email Poster  Edit Message
Well, the band is over 2 octaves, so no, Rr won't be nearly constant.

My old antenna is gone, tree fell. But this one measures ~50 ohms ground resistance and about 4 ohms radiation resistance so ~54 ohms at the low end of the band.

But, because Rg so dominates the antenna, the combination doesn't vary a lot across the band.

John Davidson

Edited by - _J_ on 10/13/2012 8:51:32 PM

gzimmerPosted - 13 October 2012 22:40  Show Profile  Email Poster  Edit Message
Golf,

If you go back to my antenna impedance graph you will see how the resistance and the reactance changes across the band.

www.qsl.net/vk3gjz/Misc/ZimAntGraph.JPG

Depending on the antenna length, the reactance normally only gets to zero at two places, the 1/4 wave and 1/2 wave points.

At 1/4 wave the resistance is 75 Ohms or so, and at 1/2 wave the R is many K ohms.

(hard to see low value R on the scale of this graph)

At all other points, if you tune out the reactance, then only antenna resistance is left.

........ Zim


Edited by - gzimmer on 10/13/2012 11:24:40 PM

golfguruPosted - 13 October 2012 23:21  Show Profile  Email Poster  Edit Message  
I'm embarrassed to say I can't find it Zim.
I know I have downloaded it twice already and was loath to have to admit it.
Thanks for the link.

That's what I was after - one should be able to match it "once only" to the ground R or match it closely at the low end of the band which is where the biggest power boost is needed?

....................

Edited by - golfguru on 10/13/2012 11:56:23 PM

gzimmerPosted - 13 October 2012 23:24  Show Profile  Email Poster  Edit Message
have posted it, see above
golfguruPosted - 13 October 2012 23:43  Show Profile  Email Poster  Edit Message  
Have been busy editing my post as well - see above.

................

gzimmerPosted - 13 October 2012 23:57  Show Profile  Email Poster  Edit Message
> one should be able to match it "once only" to the ground R or match it closely at 530kHz which is the frequency that needs the biggest power boost?

Yes, some sets do this, the ones with a small fixed coupling coil on the input. But it means that the set is hot at one point, and lousy everywhere else.

And it gets a lot more complicated when you realise that there will be strong double humping between the antenna and the tuned circuit.

....... Zim

gzimmerPosted - 14 October 2012 0:11  Show Profile  Email Poster  Edit Message
John,

> ...note all T-lines and antennas are tuned --to something.

I'm being pedantic, but I see a big difference between "un-tuned lines" (eg coax), and "tuned lines" (eg open feeders).

Coax is only happy when it has the correct resistive termination. If you depart from this you get high SWR, which means high losses.

Tuned lines however are often used with very high SWR. They can have thousands of volts at the voltage nodes. Because they have air as the insulator (and thick conductors), they exhibit little loss even with high SWR.

Tuned lines can tolerate both Resistive and Reactive mismatches. Coax cannot.

In the early days, Tuned Lines were common. It wasn't till after WW2 when coax became readily available, that transmitters and antennas were designed to match 50/75 Ohms.

People these day have forgotten how to use Tuned Lines. They are very simple and unforgiving and have low loss.

But then you know all this.

....... Zim

Edited by - gzimmer on 10/14/2012 12:13:56 AM

gzimmerPosted - 14 October 2012 0:36  Show Profile  Email Poster  Edit Message
Re-reading the posts, it seems there is confusion about tuning an antenna via a feed-line using an ATU at the bottom.

To put it simply, you can't do this with Coax because any mismatch at the top will cause high SWR on the coax which causes losses.

But you can do this with open-wire feeders (Tuned Lines). Because open-wire feeders are tolerant of high SWR, you can correct any antenna/feeder mismatch by "tuning out" the reactance at the bottom with an ATU.

As John says, a mis-matched feed-line itself will transform the impedance to some other value, so certain antennas and feed-line lengths will be more convenient than others.

Again the classic is the 1/2 wave Zep antenna. It is fed at one end (which is a very high resistance) using open-wire feeders, so the SWR in the feed-line is very high. But the feed-line length is usually chosen to be a 1/4 wave, which neatly transforms the high R at the low to a low R at the bottom.

It's the logical equivalent of extending the antenna from 1/2 wave to 3/4 wave. Except the feed-line doesn't radiate or pick up noise.

........ Zim

Edited by - gzimmer on 10/14/2012 12:41:59 AM

Garry NicholsPosted - 14 October 2012 11:35  Show Profile  Email Poster  Edit Message
Hey golf',

Hope I'm not being elementary, but I think that it has not been mentioned that for a fixed length end fed wire, the feedpoint Rr will vary as the frequency is changed. It goes down as frequency goes down and up as frequency goes up. That is because the wire becomes shorter (in terms of wavelength) as the frequency is lowered, and vice-versa.

The amount of capacitive reactance will go up as the wire becomes shorter wrt wavelength as frequency is lowered.

So the wire can't be successfully tuned and matched just once for one place on the band and be resonant and matched over the whole band.

Bob Dylan was right. The constants they are a changin . . . .

Zim and John;

Interesting discussion! I have assumed that the twin that you split was TV twin, Zim. Seems like at MW the plastic quality would not have been an issue. Isn't dielectric loss greater as frequency goes up?

Also, it would seem fruitless to try to extend DX listing range by increasing the vertical run of an antenna to get lower angle pickup because you say that the low angle sigs are too weak for a crystal set to deliver to the ear.

I guess that means the the DX we hear (on our typically more horizontal than vertical wires) is the result of relatively high-angle multi-hop propagation?

So filtering out interfering locals by minimizing vertical runs of our antennas would be the strategy for DX?

Garry

Edited by - Garry Nichols on 10/14/2012 11:46:10 AM

Edited by - Garry Nichols on 10/14/2012 11:49:25 AM

_J_Posted - 14 October 2012 14:19  Show Profile  Email Poster  Edit Message
Didn't understand what you intend to imply about coax v. open line (I assume you mean twinlead). Besides coax being unbalanced and twinlead and "ladder line" being balanced, I know of no difference being tunable (i.e. matchable), SWR tolerance etc. They are both just T-lines, like Quad lines, wave guide and others.

John Davidson

gzimmerPosted - 14 October 2012 15:19  Show Profile  Email Poster  Edit Message
John,

To be fair, you could say that all T-lines have higher losses at higher SWR, but that open-wire feeders have such low loss to begin with, that even with high SWR the losses are small. With coax the normal losses are significant and with higher SWR become prohibitive.

There are lots of references: This one has a few graphs and figures.
http://www.g4ilo.com/antenna.html

So the bottom line is that coax is intolerant of SWR as its losses quickly rise as SWR increases. On the other hand, open-wire is extremely tolerant of SWR. It is often operated with very high SWR.

Which effectively means that you can tune an antenna via open line but not via coax. With coax the antenna must be resonant and can only be operated over a narrow range of frequencies. With open wire line you can put up most anything and have it work efficiently by tuning it at the bottom.

.......... Zim

Edited by - gzimmer on 10/14/2012 5:55:52 PM

Steve McDonaldPosted - 16 October 2012 19:5  Show Profile  Email Poster  Edit Message  
An interesting discussion...are you feeding the parallel line directly into your detector circuit...one side to ground and the other to the tuner input or ??

Steve / VE7SL

Topic is 5 Pages Long:    1 2 3 4 5

Click Here To Close Thread, Administrators & Moderators Only.

Show All Forums | Post Reply