UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 13th Feb 2018, 7:56 am   #41
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
Default Re: Valve AM transmitter (very very low power!)

That is strange, when I follow the link at the bottom of post #40 I get logged out of the forum.

Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is offline  
Old 13th Feb 2018, 8:12 am   #42
Boater Sam
Dekatron
 
Boater Sam's Avatar
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,545
Default Re: Valve AM transmitter (very very low power!)

No, you are not. Its an old thread therefore the header to the page is the one you would see if you just visited. If you open in a new tab you will then see your original logged in header and the new one.
__________________
Boater Sam.
BVWS Member
Boater Sam is offline  
Old 13th Feb 2018, 9:32 am   #43
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
Default Re: Valve AM transmitter (very very low power!)

I dont get logged out if I follow this link https://www.vintage-radio.net/forum/...d.php?t=143862
But I do if I follow this one https://vintage-radio.net/forum/showthread.php?t=139297
I dont understand.
Mike

edit, just realised the one that takes you to a "logged out" page is minus the www.
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum

Last edited by crackle; 13th Feb 2018 at 9:42 am.
crackle is offline  
Old 13th Feb 2018, 10:05 am   #44
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 6,152
Default Re: Valve AM transmitter (very very low power!)

I don't get logged out by clicking on either of those links, Mike, though I have on occasion linked to pages which ask for my log-in details, even though I'm already logged in. Perhaps one of the mods. can explain the reason for this.
'LIVEWIRE?' is offline  
Old 13th Feb 2018, 10:12 am   #45
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,556
Default Re: Valve AM transmitter (very very low power!)

The site with the www at the start has a different "session" to the one without the www. You're only logged into one of them. You haven't been logged out.
MrBungle is online now  
Old 13th Feb 2018, 11:09 am   #46
boxdoctor
Heptode
 
boxdoctor's Avatar
 
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 583
Default Re: Valve AM transmitter (very very low power!)

I bow to Argus25's greater knowledge on this matter. I have done little theory on aerials, and the terminology in this particular case is, to me, somewhat ambiguous. Tony.
__________________
The job "couldn't be done".
He didn't know that,
So he did it.
boxdoctor is offline  
Old 13th Feb 2018, 12:01 pm   #47
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,449
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by kalee20
I'm guessing that aerial capacitance increases with wire diameter, and when you do the series-parallel equivalent transformations you get the variation laws you describe?
Yes, capacitance increases with wire diameter. The length^2 variation of the series resistance comes from considering radiation. The 1/length^4 variation of the parallel resistance comes from a combination of the series variation, the capacitance (varies something like length, for a short antenna) and then the series-parallel transformation.
G8HQP Dave is offline  
Old 13th Feb 2018, 12:05 pm   #48
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,449
Default Re: Valve AM transmitter (very very low power!)

The different behaviour of two 'equivalent' forum links seen by different people could depend on whether they have ticked the 'keep me logged in' box.
G8HQP Dave is offline  
Old 13th Feb 2018, 11:54 pm   #49
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,580
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by boxdoctor View Post
I bow to Argus25's greater knowledge on this matter. I have done little theory on aerials, and the terminology in this particular case is, to me, somewhat ambiguous.
I bow to both Argus25 and G8HQPDave on the aerial theory! Thanks lads!
kalee20 is offline  
Old 14th Feb 2018, 3:04 am   #50
usradcoll1
Pentode
 
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 203
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by coopzone View Post
Someone asked about the size of the copper wire i used, i think it was around 32/34 swg but i don't think it matters to much, my first attempt was using a single strand of rj45 cat 6 network cable (probably around 26swg) and tha worked ok - just did not look very pretty!
I just finished winding the one coil that requires only 35-45 turns.
The only wire I had on hand was insulated 28ga awg solid core hook-up wire. My home made coil form is about 60mm long and about 31mm in diameter. My 35-45 turn coil takes up about 55mm of the coil form.
Naturally, the 65-75 turn coil is going to take up more room on my coil form. I can fabricate a longer form that will accommodate more turns.
Can these two windings be slightly apart or do they have to close together, as shown in the picture? Please advise!
usradcoll1 is offline  
Old 14th Feb 2018, 9:37 am   #51
coopzone
Tetrode
 
coopzone's Avatar
 
Join Date: Jan 2018
Location: Stafford, Staffordshire, UK.
Posts: 61
Default Re: Valve AM transmitter (very very low power!)

I'm not really an expert on coil winding, but i don't think it's to crucial at this sort of frequency. The overall length of my coil is around 85m. It's meant to be one coil centre tapped (it's not in the centre though! closer to 1/3rd ) so keeping the winding next to each other would be preferable. You may need to increase the number of turns on the larger coil since they are slightly further apart due to the insulation. You can find a formula / calculator for air wound coils on the googletron! (someone - back up the thread calculated the micro-henries for the coils - so you may want to aim at those).

To be honest i just guestimated it, based on previous experience of building crystal set's. The best way to be sure is to measure the frequency using a scope or DVM (i got one from China, cost #9.50 and does frequency up to 20mhz along with V/A, AC/DC and ohms)

Just experiment, that's the fun! You could for example put the smaller winding on top of the bigger winding, as long as you keep the direction the same (ie the correct phase for the feedback to make it oscillate)

Sorry i can't be more helpful.

coop
coopzone is offline  
Old 15th Feb 2018, 7:12 am   #52
Argus25
Octode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,660
Default Re: Valve AM transmitter (very very low power!)

One thing that might be worth noting;

The impedance of space or a vacuum , said to be 377 ohms and cited in the equations in post # 5 on the other thread, is the wave impedance in the far field from an antenna and is the ratio of the electric field E in volts per meter, to the magnetic field H in amps per meter.

However, in the near field to a transmitting antenna (and all pantry transmitters and their listening radios work in the near field because of the transmitted wavelength, say 300m is much longer than the geometry of a house) the impedance is not necessarily 377 Ohms.

For a whip antenna, the near field is largely electric in nature near the antenna with a small magnetic field component, so the E to H ratio or wave impedance there is high, but for a magnetic or loop antenna the magnetic field is dominant and the electric field low and the wave impedance lower.

Most likely it would be the case then, that the loop antenna would be better for pantry transmitters if the receiving radios had ferrite rods, such as transistor radios, but for valve radios with a wire antenna, then a wire transmitting antenna would be better. That would be an interesting experiment.

In the far field where the E to H ratio has settled on 377 ohms, it would likely make little difference and probably one could not tell whether the transmitted signal originated from a dipole antenna or a magnetic loop.
Argus25 is offline  
Old 15th Feb 2018, 5:40 pm   #53
usradcoll1
Pentode
 
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 203
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by coopzone View Post
Just experiment, that's the fun! You could for example put the smaller winding on top of the bigger winding, as long as you keep the direction the same (ie the correct phase for the feedback to make it oscillate)
Thanks! I still have to gather all the parts and finish the coil. I wrote down all the latest revisions.
I haven't built anything from scratch in ages. Should be interesting.
usradcoll1 is offline  
Old 16th Feb 2018, 12:10 pm   #54
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,449
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by Argus25
Most likely it would be the case then, that the loop antenna would be better for pantry transmitters if the receiving radios had ferrite rods, such as transistor radios, but for valve radios with a wire antenna, then a wire transmitting antenna would be better. That would be an interesting experiment.
Yes. I find that the distinction between near-field and far-field does not seem to be sufficiently appreciated, even by some who think they know about antennas.
G8HQP Dave is offline  
Old 19th Feb 2018, 1:29 am   #55
Aubade65
Triode
 
Join Date: May 2012
Location: Oxfordshire, UK.
Posts: 22
Default Re: Valve AM transmitter (very very low power!)

There is an interesting web page at http://www.electronbunker.ca/eb/antennamatch.html that shows in relatively simply terms how a short antenna for a pantry tx can be built and the basic theory behind it
Aubade65 is offline  
Old 19th Feb 2018, 2:36 am   #56
rambo1152
Heptode
 
rambo1152's Avatar
 
Join Date: May 2010
Location: Manchester, UK.
Posts: 844
Default Re: Valve AM transmitter (very very low power!)

The URL is case sensitive after the first /

http://electronbunker.ca/eb/AntennaMatch.html
__________________
--
Graham.
G3ZVT
rambo1152 is offline  
Old 20th Mar 2018, 1:23 am   #57
usradcoll1
Pentode
 
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 203
Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by usradcoll1 View Post
Thanks! I still have to gather all the parts and finish the coil. I wrote down all the latest revisions.
I haven't built anything from scratch in ages. Should be interesting.
I finally finished this project! I'm pleasantly surprised how well it works. Better than the others I built ages ago.
I still have a little perfecting to do. The audio quality is a little distorted, but it could be caused by the amount of line output that my CD puts out
As said, a little experimenting will take place.
usradcoll1 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:37 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.