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Old 12th Feb 2018, 12:04 pm   #21
Chindit
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Default Re: Valve AM transmitter (very very low power!)

Well my Wireless Set 18 is supposed to put out 1/4 watt and that will go about 5 or 6 miles with a 12 foot whip aerial.
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Old 12th Feb 2018, 12:29 pm   #22
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Default Re: Valve AM transmitter (very very low power!)

That's good. What mode? Is the rx in an electrically quite area?

Getting a good AM signal in an urban environment with a s/n ratio acceptable to the average listening public is quite a challenge with 1W ERP and only 30ft of transmitting antenna. The signal can be lost in the noise only a couple of miles away. Then there's also the problem of building absorption across the service area, but the station can be almost arm-chair quality 10 - 15 miles away out in the county.

(This post relates to the previous mention of legal RSLs, signal strengths and interference.)

Last edited by Ian - G4JQT; 12th Feb 2018 at 12:48 pm. Reason: clarity
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Old 12th Feb 2018, 12:41 pm   #23
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Default Re: Valve AM transmitter (very very low power!)

No discussion of broadcasting in this thread please.

Users of pantry transmitters do have to be very careful about range in the UK. Houses tend to be close together and one of your near neighbours may be an AM radio enthusiast. Ofcom have better things to do than chase harmless microtransmitter users, but they will take enforcement action if they receive a complaint. If your neighbours can't hear your signal then there won't be any complaints.

Quite apart from the legal aspects, you won't make many friends by making a nuisance of yourself.
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Old 12th Feb 2018, 2:03 pm   #24
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Default Re: Valve AM transmitter (very very low power!)

A full 250mW seems an awful lot. Even with super lossy matching to the aerial, there is plenty of room for distortion to manifest itself as spurious harmonics. I'd want to have a good listen around with a battery portable wide-coverage receiver, especially at multiples of the transmitter frequency and multiples of the wavelength implied by the aerial.
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Old 12th Feb 2018, 2:05 pm   #25
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Default Re: Valve AM transmitter (very very low power!)

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Originally Posted by paulsherwin View Post
Users of pantry transmitters do have to be very careful about range in the UK. ....you won't make many friends by making a nuisance of yourself.
One saving grace here is that practically every pantry transmitter setup I have seen on this forum has such a low actual radiated power that it is highly unlikely any reasonable level signal would make it to a neighbor's house. Even ones described have had difficulties getting to all the rooms in the one house without falling into the noise or getting dominated by mains interference.

There are two fundamental reasons for this, and it is not related to what would be regarded in some countries as a legal 100mW transmitter circuit. The transmitter circuits are often capable of at least 100mW output (into a matched load) It is the physical antenna size and matching that is the issue.

Very basically the actual radiated power level is proportional to the product of the radiation resistance of the antenna and the rms antenna current(squared).

Because the wavelength of the transmission frequency is very long compared to the wire antenna length the radiation resistance of the wire is very very low at MW band frequencies.

Also it is difficult to establish any reasonable RF current in the antenna at these frequencies for a 10 ft antenna, the only way to resonate it is with a loading coil where the dominant reactance is in the coil, not the antenna, which makes it a poor radiator.

It is possible to increase the RF current in the whip antenna if high voltages are applied to the antenna, or the antenna is at least 20 ft long or more and properly loaded to achieve the same effect. Or forming the antenna into a closed loop of at least 1 to 2 meters diameter (preferably with more than one turn) and resonating it to increase the currents in the conductors that comprise the antenna that way.

This explains why the valve pantry transmitter is a little better than its semiconductor counterpart, because the rms voltage applied to the wire antenna is greater from the plate's tank circuit and so the rms current in the antenna wire is higher, compared to a lower voltage transistor tank circuit, for the equivalent antenna length.

It is also worth remembering that the radiation resistance of an antenna is proportional to the 4th power of the frequency (for a loop and to the square of the frequency for a whip), so for example, all other things equal, trying to transmit on 600kHz could have about 1/16 the transmission range as you would get on 1200kHz for the same loop antenna, so its much easier to get better results transmitting near the top end of the AM band when the antenna is limited. Or say a transmitter that was operating on 3MHz could seem spectacularly better than one on 1500kHz for the same sized antenna.

Last edited by Argus25; 12th Feb 2018 at 2:29 pm. Reason: clarify
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Old 12th Feb 2018, 2:27 pm   #26
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Default Re: Valve AM transmitter (very very low power!)

Hello,

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've attached a slightly better picture of the coil.

And, sorry to remind folks, but i did say i missread the power ouput and at best it's around 25-30mw not 250mw.

In real terms with about 1.5 metre areal it will transmit within the same room aound 10ft


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Old 12th Feb 2018, 2:34 pm   #27
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Default Re: Valve AM transmitter (very very low power!)

I could be wrong, but I think the type of variable capacitor you have used is a transistor radio type, for very low voltage applications. It might be ok but the higher voltages in the plate circuit might cause its dielectric to break down, but like I say I might be wrong about this, but be ready for it if it shorts out.
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Old 12th Feb 2018, 2:40 pm   #28
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Default Re: Valve AM transmitter (very very low power!)

Those plastic capacitors are quite reliable at reasonable voltages. I've seen people using them in low power level ATUs. Not sure what they are officially rated at though.
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Old 12th Feb 2018, 2:55 pm   #29
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Default Re: Valve AM transmitter (very very low power!)

It may be safer to fit a cap in series.
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Old 12th Feb 2018, 3:53 pm   #30
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Default Re: Valve AM transmitter (very very low power!)

25mW certainly sounds less worrying than 250. Probably still worth having a listen about for unwanted signals, though, if you have a suitable portable receiver.

If you are on good terms with your neighbours, they are less likely to think ill of you. So be open about your "naughty" little transmitter; show them it in action even, and explain you're as keen as them not to be causing interference to anything they may be watching or listening to.
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Old 12th Feb 2018, 4:23 pm   #31
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Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by Argus25 View Post
Because the wavelength of the transmission frequency is very long compared to the wire antenna length the radiation resistance of the wire is very very low at MW band frequencies.
Surely the radiation resistance of the antenna is very high due to it's short length, and this is why it does not draw much power from the transmitter, in the same way that a high impedance pair of headphones will draw less power from an amplifier than low impedance ones?
In other words, it is not loading the transmitter as would a matched load of lower impedance. Tony
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Old 12th Feb 2018, 4:29 pm   #32
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Default Re: Valve AM transmitter (very very low power!)

The impedance is high, but the radiation-resistance (which essentially expresses how well the antenna is matched to the ether) is low.

See https://www.w8ji.com/radiation_resistance.htm for a good exposition.
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Old 12th Feb 2018, 5:11 pm   #33
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Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by boxdoctor View Post
Surely the radiation resistance of the antenna is very high due to it's short length, and this is why it does not draw much power from the transmitter...
I had the same question a few months ago! I was thinking of the whip aerial as being a capacitive reactance in parallel with a resistance. Argus was thinking of it being a capacitive reactance in series with a resistance.

So for almost no power, the resistor for you and I would be very high. For Argus it would be very low.

Each is valid, of course, though I prefer to consider the parallel case (because it's easy to measure the RF voltage on the aerial, whereas difficult to measure the RF current being fed to the aerial).
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Old 12th Feb 2018, 6:51 pm   #34
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Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by Argus25 View Post
That is exactly right, even if the output stage could deliver 250mW into a matched dummy load there is no way the radiated power would be anything like 250mW, the radiation resistance of 10 foot of wire is abysmally low at MW band frequencies. Probably the radiated power would be less than 10mW from that wire if you were lucky.
Useful advice, Hugo, and I recall your design in this post:

https://www.vintage-radio.net/forum/...6&postcount=36

Hello OP (Coopzone) : here is a design built by Argus25 with great success. If you read through the whole thread, you'll see some very useful wider discussion relating to the problems matching the output impedance of most home-brew pantry transmitters to an effective antenna. As Hugo points out, most antennas are 3m or so in length, and so are not resonant at the design frequency of the pantry transmitter. Going up in frequency is a good plan, as Hugo illustrates mathematically, but there are some other great ideas in the thread containing the link I've posted.
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Old 12th Feb 2018, 10:02 pm   #35
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Default Re: Valve AM transmitter (very very low power!)

The term 'radiation resistance' specifically refers to the resistive part of a series impedance, and hence is low for a short antenna.

Roughly speaking, this series resistance varies like the length^2 for a short antenna; the corresponding (much higher) parallel resistance varies more like 1/length^4. In addition, the series version is almost independent of antenna thickness and so easy to estimate, while the parallel version is strongly dependent on thickness and more difficult to estimate. This is why the series version is more commonly used.
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Old 12th Feb 2018, 11:21 pm   #36
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Default Re: Valve AM transmitter (very very low power!)

That's really useful info Dave (aerials and radiation are not subjects I'm clued up on).

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?
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Old 12th Feb 2018, 11:50 pm   #37
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Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by boxdoctor View Post
Surely the radiation resistance of the antenna is very high due to it's short length
Tony,

The radiation resistance I'm referring to (the definition of radiation resistance) is the resistance responsible for radiating electromagetic waves. Nothing to do with inductance,capacitance, reactances, or ohmic resistances.

The higher value it is, the more power radiated as EM waves from an antenna.

This resistance comes about due to the Abraham-Lorentz force or the recoil force on charges that are being accelerated in the antenna.

This parameter relates to the geometry of the antenna, for example for a whip antenna is is proportional to the square of the effective height of the antenna and proportional to the square of the operating frequency. For a loop antenna it is proportional to the 4th power of the frequency, the square of the number of turns and the square of the loop's cross sectional area.

Look up radiation resistance of antennas and the Abraham-Lorentz force responsible for it.

Hugo.
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Old 13th Feb 2018, 12:07 am   #38
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Default Re: Valve AM transmitter (very very low power!)

But there will be an actual resistance in the electrical sense, corresponding to the energy that goes into the electromagnetic wave.
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Old 13th Feb 2018, 12:27 am   #39
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Default Re: Valve AM transmitter (very very low power!)

Nice job! I've made a note of the circuit for future reference.

I also notice that you're using one of those generic 'mini fashion' MP3 players that are ridiculously cheap. I have been using those for some years. They have the useful feature, whether by accident or by design, whereby if switched to 'off' they actually start playing as soon as the external 5 volts is applied, and stop playing when power is removed. This makes them great for unattended background music sources, controlled by only a simple time switch.

Sorry to go OT, but it means you could start and stop your pantry system automatically!
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Last edited by Phil G4SPZ; 13th Feb 2018 at 12:28 am. Reason: Typo
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Old 13th Feb 2018, 12:44 am   #40
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Default Re: Valve AM transmitter (very very low power!)

Quote:
Originally Posted by julie_m View Post
But there will be an actual resistance in the electrical sense, corresponding to the energy that goes into the electromagnetic wave.
Yes, the radiation resistance being the part that radiates the the EM waves, which is why the radiated wave power is proportional to the (rms current)^2 times the radiation resistance. The Ohmic resistances anywhere simply turns energy into heat, not EM waves.

Since for a whip antenna, the radiation resistance being proportional to the effective antenna height (he/wavelength)^2, naturally if the height of the antenna is small compared to the wavelength, the radiation resistance is very very low, which is why short wire antennas are pretty limited for the AM radio band as good radiators, because 3 meters of wire is a very small fraction of say 300m wavelength for a 1MHz transmission.

In practice we never notice this issue working with VHF and UHF because the antenna height is usually a good proportion of the wavelength, not just 1% of it as we have with the pantry transmitter and a 10 foot wire.

PS: I previously posted the formula for radiation resistance for whips with effective height he, and loops with turns N and area A on this thread post #5. As you can see radiation resistance is thought of as virtual and also depends on the impedance of space. The formulae also explains why small coils and ferrite rods make poor radiators because of their small cross sectional area.

https://vintage-radio.net/forum/showthread.php?t=139297

Last edited by Argus25; 13th Feb 2018 at 1:01 am. Reason: add link
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