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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 6th Jun 2017, 2:40 pm   #81
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: RF signal path in this circuit...

Error, post #77.

3m=approx 1/100th of a wavelength at 1MHz, of course. I took a phone call while posting and got distracted !!
__________________
Al
Al (astral highway) is offline  
Old 6th Jun 2017, 2:44 pm   #82
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: RF signal path in this circuit...

Loop coil left untuned, feed into one end of the loop leave the other end of the loop not connected to anything, in other words treat as an end fed wire antenna that happens to look like a coil?

Lawrence.
ms660 is offline  
Old 6th Jun 2017, 3:18 pm   #83
Guest
Guest
 
Posts: n/a
Default Re: RF signal path in this circuit...

Simply use it as an end fed aerial, the extra 'top hat' capacitance will help too.
 
Old 6th Jun 2017, 5:20 pm   #84
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: RF signal path in this circuit...

Quote:
Simply use it as an end fed aerial, the extra 'top hat' capacitance will help...
So effectively, using it as an inductor for matching - but still terminated with a short antenna? It doesn't work unterminated.

I may, alternatively (because it does look attractive) put it into service as a receiving loop, either with or without amplification.

It would be a shame if the magnetic loop isn't an option for me at these low frequencies, but it does look as if all this giant and copious ferrous metalwork everywhere is messing up all the calculations.
__________________
Al
Al (astral highway) is offline  
Old 6th Jun 2017, 11:15 pm   #85
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: RF signal path in this circuit...

Quote:
Jeremy, did you use an iPhone as the audio source to mimic my trial ? Otherwise what did you use and how many mV P-P was the audio source ?
I downloaded a radio 5 live podcast and played it back via a little Eee PC netbook into the AF input. It struggled to give 100% modulation but it was good enough for a trial. The audio level was about 0.5Vpkpk from the Eee PC. I think my RF power level is 150mW PEP into the loop. This is compared to the 100mW for the standard circuit.

I guess how far it goes depends on a few things. eg how quiet the band is, how good the receiver is, what it uses as an antenna, and also what signal to noise ratio qualifies as acceptable. I think my little Flirt radio sounded clean and punchy about 10metres away. But the signal strength drops quickly after this. It can be heard quite well at further distances (eg towards 15 metres) but I can hear background interference and the Flirt radio has to be held at the ideal orientation to get the best reception. But at 10m range it sounded fine. Maybe on a higher quality radio I would have noticed some background hiss or interference at 10m range but a typical radio talk show sounded absolutely fine on the Flirt at 10m range

.
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline  
Old 6th Jun 2017, 11:44 pm   #86
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: RF signal path in this circuit...

I tried a more critical test with the radio hard pressed to my ear and tried going in various directions out from the house.

To get a signal with no subtle interference in the background the range is more like 6 metres but pretty much all of the genuine radio stations I tuned to had some subtle interference on them when the radio was pressed to my ear like this. If I was to turn on my plasma TV it would probably wreck the range in the back garden test and I think the level of interference from various modern appliances will often cripple the effective range of a tiny transmitter like this?
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline  
Old 7th Jun 2017, 12:14 am   #87
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: RF signal path in this circuit...

I tried giving it a realistic 'in house' pantry test by using my old Trio KR-2600 receiver downstairs. This is a 1970s receiver in the sense that it is a hifi amp with integral receiver. It also has a built in signal strength meter and the MW antenna is a small rod/stick antenna on the back that can be tilted at an angle for best reception.

With the pantry loop in the top corner room and the Trio receiver in the downstairs opposite corner of the house running with its stick antenna at the back the pantry transmitter was the biggest signal on the band apart from 693kHz Radio 5 live which was hard over to full scale on the s meter dial.

The pantry signal was a strong signal on the s meter, not quite full scale but 8/10 of the way over. It sounded noise free (KR-2600 amp and some old Warfedale speakers) and was just like 5 live but up at about 1.5MHz

I'd guess the diagonal distance was >5 metres. I could move the pantry loop a few more metres further away and retry but it won't be tonight...
__________________
Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 7th Jun 2017 at 12:38 am.
G0HZU_JMR is offline  
Old 7th Jun 2017, 1:36 pm   #88
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: RF signal path in this circuit...

Astral Highway,

There is a lot of info in the ARRL antenna handbook of course, but sometimes just some general issues that might help you while experimenting with your antennas/pantry transmitters.

The trick to get an antenna to radiate electromagnetic waves is to be able to accelerate and decelerate charges at RF frequencies in the conductors, in other words have the highest possible RF current.

As you know in a tank resonant circuit, the circulating current is very high inside the circuit and low outside it. This is another reason why a loop design is helpful when you are trying to transmit, on low powers, from a physical structure which is small compared to the wavelength you are transmitting on. This is because the loop is closed by the tuning capacitance, unlike a whip with one end to free space.

So the closed nature of the loop resonant circuit helps establish a good RF current in the conductors. But it needs to be supplied with energy to keep it going and overcome losses. For a loop antenna for an AM transmitter, it is important to have the main resonant circuit at the transmission frequency obviously, but you also need to effectively couple into it, without over-damping the resonant (tank) circuit and without under coupling either. It is best done with a tap at about 1 turn of say a 5 to 7 turn loop, although tapping into the tuning capacitance also works.

Ideally the input impedance of the tap into the loop at resonance is measured (it can be approximately done with a signal generator and trying series resistors and comparing the voltage across those with that of the loop input, but there are analyzers as you know), and be matched or suited to the RF output stage you have. (I stick to 50R systems to make the measurements and matching easier and also allow the use of coaxial feed-lines to the antennas, but this requires matching networks).

If one tries to use a linear conductor (whip antenna) to transmit at a frequency where the conductor length is very well below the wavelength, the usual method is to insert a loading coil at the base (or middle) and add, as suggested, a top hat on the antenna rod. In essence, the increased capacitance & inductance allows to rod to draw more current and/or allow resonance, accelerate more charges and generate more powerful EM waves.

However, for say a 1.5 to 2m long metal rod, it is very very hard at 1500kHz to get any significant antenna current. An air cored loading coil to improve this has to be physically massive, and a capacitive top hat about a meter or more in diameter.

I did find a way to obtain a good antenna current for a 1.5 to 2m long whip at 1500kHz, but its not the kind of thing normally described in antenna handbooks. I realized that the only satisfactory way to gain a good RF current, was to "go all Nicola Tesla on it" and transform the drive voltage to the antenna base up very high. I've attached a picture of what is a successful 50R input impedance 1.5M long transmitting whip antenna at 1400kHz. The bandwidth is also fine for music. The pot-core in its base is an auto-transformer. It results in over 150V at the base of the antenna, and easily lights the current limited neon bulb in the assembly. This sort of thing comes close to, but not as good as the 50R loops I built.(the brown cylinders on either side of the pot-core are made of tufnol)
Attached Thumbnails
Click image for larger version

Name:	XMTR3.jpg
Views:	75
Size:	44.9 KB
ID:	144095   Click image for larger version

Name:	XMTR1.jpg
Views:	76
Size:	86.0 KB
ID:	144096  

Last edited by Argus25; 7th Jun 2017 at 1:44 pm.
Argus25 is offline  
Old 7th Jun 2017, 6:48 pm   #89
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: RF signal path in this circuit...

Quote:
Originally Posted by Argus25 View Post
I did find a way to obtain a good antenna current for a 1.5 to 2m long whip at 1500kHz, but its not the kind of thing normally described in antenna handbooks. I realized that the only satisfactory way to gain a good RF current, was to "go all Nicola Tesla on it" and transform the drive voltage to the antenna base up very high.
Ahah! That is one giant coincidence! Pictured is the secondary of the secondary coil from the Tesla coil I was constructing a while back, which is on hold for the moment. It's built like a tank for 1.5kW RF. It just happens to be resonant at around 1MHz!

I tried the coil base fed with a short length of coax. It is (see pic) resting on a copper ground plane and tuned by a 0-220pF variable capacitor. The results are startling. With a 3m wire at the top there, I get a huge signal. The scope trace shows the modulated signal, picked up by the scope probe around 2metres away from the antinode, hanging in the air. I will wait until dark to see if I can get any luminescence from an old glow tube I have at hand. The Q of that coil is 650, so I expect so. I haven't checked the range at but 6 metres away even at mid-volume control level on the receiver things are really loud and clear.

The signal at a receiver has a strong carrier but is undermodulated when fed by line level audio from my iPhone. Line level from an iMac is perfect (It's about double, by my measurements.) Music is clear and the signal to noise ratio, by ear, is excellent. Instead of messing around with gain in my modulator, I will likely reduce the supply voltage to 18V. It is currently 22.5V. This will increase the modulation percentage but won't require me to adjust the DC bias conditions of the transistors.

The secondary coil is quite cumbersome and although good for a lash-up, not sure if it can be a permanent fixture. It will need some thought to fit in with the aesthetics of my place... But it was a lot of work to make the thing and if it can double as a base load as well as a Tesla coil secondary that's double value. I designed it (around a year ago) to be easily unscrewed by twisting the whole thing - it has hidden plumbing fittings underneath.

Quote:
Originally Posted by Argus25 View Post
I've attached a picture of what is a successful 50R input impedance 1.5M long transmitting whip antenna at 1400kHz. .. It results in over 150V at the base of the antenna, and easily lights the current limited neon bulb in the assembly. This sort of thing comes close to, but not as good as the 50R loops I built.(the brown cylinders on either side of the pot-core are made of tufnol)
That's a beautiful piece of work. Really nice crafstmanship there!
Attached Thumbnails
Click image for larger version

Name:	untitled-1587.jpg
Views:	74
Size:	54.1 KB
ID:	144122   Click image for larger version

Name:	untitled-1588.jpg
Views:	70
Size:	53.0 KB
ID:	144123  
__________________
Al

Last edited by Al (astral highway); 7th Jun 2017 at 6:55 pm.
Al (astral highway) is offline  
Old 7th Jun 2017, 9:22 pm   #90
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: RF signal path in this circuit...

Update:

The configuration in the last photo wasn't quite right. The top load is now (correctly) at the distal end of the 3m wire which terminates the base-loading coil.

As you can see from the scope trace, the scope probe hanging in the air metres away from the coil is picking up a beautifully modulated signal of higher amplitude than before and better modulation depth than before.

The signal is now much stronger and the volume on my old Marconi is only turned up a fifth of the way.

Six (6) metres away, the scope probe hanging in the air with a 20centimetre aerial demodulates the signal and shows the audio component with a signal strength of 8mV P-P, in case anyone would like to work out what that means for the signal strength at the antenna!?

The quality of the sound is better by far than I can receive on any AM station, seriously!

I don't think I need to add treble lift at all. Also to make sure that pointless signal components weren't taking up bandwidth, I did knock out some bass in the first few octaves by reducing the input capacitor to 1uF instead of 10uF (it obviously forms a high-pass filter together with the bottom -3.3K-- resistor of the voltage divider pair in the base of the first transistor).

A 1980's Sony tuner doesn't resolve the signal nearly as well, although it shows as a signal strength of 4/5 on the meter. I haven't tried it on any portable sets yet.

In case you're interested, that top load is a 'gazing ball' from a garden centre. It was described as 'copper',. but I knew it wasn't. I burnt off the copper paint by bluing the stainless steel ball on the gas cooker!
Attached Thumbnails
Click image for larger version

Name:	untitled-1594.jpg
Views:	59
Size:	34.9 KB
ID:	144140  
__________________
Al

Last edited by Al (astral highway); 7th Jun 2017 at 9:47 pm.
Al (astral highway) is offline  
Old 7th Jun 2017, 10:22 pm   #91
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: RF signal path in this circuit...

That's a really interesting arrangement with the Tesla coil.

There was a simple thing I forgot to mention. It applies to coupling power into or out of Tank/resonant circuits like loops and especially applies to this AM loop situation. Whether using a tap of one turn or two turns of the main resonant circuit, or say coupling with a link coil as in "link coupling". It is always far better if the inductance in the coupling circuit & wiring itself is neutralized at the resonant frequency. This is easily done with a series coupling capacitor, made adjustable initially at least, to find the value that minimizes the input impedance and peaks the current. This allows the transfer of more energy into the loop from the output amplifier. (At resonance with say a 5 turn loop, even the impedance of the lower turn is still quite high and awkward, also it doesn't work as well if less than one turn is used to lower the input impedance, so it really needs the capacitor, the one for that orange loop as you can see turned out to be 240pF, but it also needed the transformer to get id down to 50R)
Argus25 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:35 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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.