TheRadioBoard

Forum for the homemade radio builder. Newbies and Experts and everyone else are welcome here!
It is currently Sat May 18, 2013 10:22 am

All times are UTC




Post new topic Reply to topic  [ 9 posts ] 
Author Message
 Post subject: tinfoil antenna
PostPosted: Thu Nov 08, 2007 9:27 am 
Offline
User avatar

Joined: Thu Jul 19, 2007 9:07 am
Posts: 469
Location: The Netherlands
Due to limited space the maximum length of my antenna can only be 5meters (16.5 feet) on the attic of my house. Last night I did some experiments to see if I could improve my antenna. As you all know with crystal radios every pW counts.

At 1MHz a 5 meter antenna is only 1/60 of the wavelength and so short that it behaves like a capacitor so it only receives the E-field. Increasing the area of the antenna should increase capacity and catch more E-field yielding more power from the antenna. To increase the antenna area I added a 5meter long and 30cm wide tinfoil sheet to the antenna wire. As expected the capacity increased and I had to retune the antenna tuner/matching unit. To my surprise the signal strength didn't increase :( .

Next I tried a second wire parallel to the first (tinfoil removed) at a distance of 90cm (3 feet). This again increased the antenna capacity but now I had about a 30% increase in signal strength :) . Next I added two more wires with each wire separated by 90cm. Again more antenna capacity and over 100% increase in signal strength over a single wire :D . Apparently each wire gives about 30% increase

In both tinfoil and wire experiments the antenna capacity increased but only by adding more wires I was able to increase the signal strength.
I don't understand why this is the case any suggestions? :?

Corne


Top
 Profile  
 
 Post subject:
PostPosted: Thu Nov 08, 2007 5:02 pm 
Offline

Joined: Sun Nov 12, 2006 6:09 pm
Posts: 1304
Location: N 64º 41' E 21º 14'
eddy currents in the foil is my guess.


Top
 Profile  
 
 Post subject:
PostPosted: Fri Nov 09, 2007 12:04 am 
Offline

Joined: Tue Jul 03, 2007 5:06 pm
Posts: 910
Location: Sandpoint, Idaho USA
Remember that antennas are black magic! But what you were doing is possibly lowering the total impedance of the antenna at the feed point which was showing up a better match to your receiver. You were also increasing the "capture area" of the antenna, and that would also explain better signals being received.
Curt

_________________
Curt, N7AH
CW forever
Connoisseur of the cold 807


Top
 Profile  
 
 Post subject:
PostPosted: Fri Nov 09, 2007 1:54 am 
Offline
User avatar

Joined: Tue May 01, 2007 12:18 am
Posts: 2069
Location: Vieques, PR
I'll make a stab at this. This is only my conjecture, not an attempted explanation to the 'black magic'.

I don't think the 30 cm wide tinfoil really adds anything to capture area in that small of a length at BCB. Yes, it would add capacitance but capacitance=signal strength is not equated.

The additional wires at 90 cm apart DO add some capture area in comparison. They are distinct and they add signal differently than one single solid conductor (like the foil) would. They are only a tiny bit out of phase so they should add.

Look at the old 1920s MW antennas. Cages and multiple conductors spaced a few feet apart. Modern explanation often tends to dismiss that as simply a large top hat that provides capacitance for the vertical 'lead in'. If V/H polarity comes into question it should be noted that most early BC stations used a similar horizontal configuration.


-Bill


Top
 Profile  
 
 Post subject:
PostPosted: Fri Nov 09, 2007 9:30 am 
Offline
User avatar

Joined: Thu Jul 19, 2007 9:07 am
Posts: 469
Location: The Netherlands
If the collecting area has influence on signal strength I should get more signal from two wires at 180cm apart than I would from 90cm. So I tried two wires 180cm apart. I had to retune the antenna tuner about the same as I would for two wires at 90cm. So the antenna capacity for two wires at 90cm and 180cm are about the same. The increase in signal strength was 30% so exactly the same as with two wires 90cm apart. Adding the wire at 90cm, so now I have 3 wires spaced at 90cm, increased the signal strength by about 70% compared to a single wire.
In all the experiments I did I got about 30% increase for each additional wire if the spacing is 90cm.

Quote:
Curt Reed wrote:
Remember that antennas are black magic!

Sure seems that way.

I'm happy that I found a way to significantly increase signal strength without increasing the antenna length.
But I'll have to look into the matching between the antenna and receiver to see if I can optimize it.

Corne


Top
 Profile  
 
 Post subject:
PostPosted: Fri Nov 09, 2007 2:12 pm 
Offline

Joined: Wed Nov 08, 2006 7:58 pm
Posts: 80
Corne,

I run a "clothesline" antenna. 4 wires at 60 feet long spaced about 3 feet apart. They work good if you have a short space and there is no way to go longer.

There must be a diminishing return at some point. The last wire I put on did not give that much more of a result so I stopped adding. It would be interesting to see where it actually stops.

DB


Top
 Profile  
 
 Post subject:
PostPosted: Fri Nov 09, 2007 2:35 pm 
Offline
User avatar

Joined: Thu Jul 19, 2007 9:07 am
Posts: 469
Location: The Netherlands
Darryl,

I see you are using a similar spacing than I do (3feet=90cm) only your antenna is 3.7 times long than mine.

Quote:
There must be a diminishing return at some point. The last wire I put on did not give that much more of a result so I stopped adding.


With me the 4th wire added about the same as the 3rd.

Quote:
It would be interesting to see where it actually stops.


I'm not going to find out where it stops if it does at all because I only have space for 6 or 7 wires before my attic space runs out.

Corne


Top
 Profile  
 
 Post subject:
PostPosted: Sat Nov 10, 2007 1:34 am 
Offline

Joined: Wed Nov 08, 2006 7:58 pm
Posts: 80
Corne,
I have enough problems with spider webs in the attic!

If I had 6 wires in my attic, I'd have to have the fire department come in and cut me loose after I got tangled up in it! :oops:

Mine is on the roof. and looks like a clothes line from the back yard.

DB


Top
 Profile  
 
 Post subject:
PostPosted: Mon Nov 12, 2007 8:41 am 
Offline
User avatar

Joined: Thu Jul 19, 2007 9:07 am
Posts: 469
Location: The Netherlands
This morning just after I replied to an email about the subject it hit me. I shouldn't look at the voltages but the power generated by the voltages.
Yesterday I redid the multi wire antenna measurements trying to understand what was going on. With one antenna wire I got 73mV. Adding the second wire increased the signal to 103mV, the third wire got it up to 124mV and the fourth wire gave 149mV.

The load during the measurements was 200k so
0.073^2/200k= 27nW
0.103^2/200k=53nW
0.124^2/200k=77nW
0.149^2/200k=111nW

Each wire adds about 26nW. Finally this makes sense.


Corne


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group