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Antenna matching

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Vaibhav Birit

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HI,
I built a 14Mhz and 4W Transmitter for CW. I connected a dipole antenna which is made of ordinary wire. it is 5 metres on both side. The link between the transmitter & antenna is through a coaxial cable, about 1 metre long.

When i measure the voltage at the point where my Transmitter connects to the coaxial cable, the voltage is found to be greater than 1000v which is the max. range of my multimeter. Actually the voltage Toggles High & Low. On high it reads 1000V . On low it reads some 300V

For measuring voltage, i user a diode capacitor rectifier & connected its output to the multimeter.

Could anyone tell me whether this behaviour is normal or whether there is any problem with my antenna matching. In that case what should my Multimeter read.
 
Vaibhav Birit said:
HI,
I built a 14Mhz and 4W Transmitter for CW. I connected a dipole antenna which is made of ordinary wire. it is 5 metres on both side. The link between the transmitter & antenna is through a coaxial cable, about 1 metre long.

When i measure the voltage at the point where my Transmitter connects to the coaxial cable, the voltage is found to be greater than 1000v which is the max. range of my multimeter. Actually the voltage Toggles High & Low. On high it reads 1000V . On low it reads some 300V

For measuring voltage, i user a diode capacitor rectifier & connected its output to the multimeter.

Could anyone tell me whether this behaviour is normal or whether there is any problem with my antenna matching. In that case what should my Multimeter read.

You can't measure this with a multimeter - as you are probably already aware, 1000V is total nonsense - 4W into 50 ohms is only about 14V.

What you need is an SWR (Standing Wave Ratio) meter, this measures the power going up to your aerial, and the amount reflected back into the transmitter. A 'perfect' aerial would reflect none back, but this is not possible in practice - you adjust the aerial to give the lowest reverse reading you can.

If you're a radio amateur, as presumably you are using the 14MHz amateur band?, you should already know all about this - it should have been part of your training and examination for a licence!.
 
If you have the diode, capacitor etc wired correctly as a diode probe, you should be able to measure the RF voltage without any problem, and from there calculate the power. W = V*V/R.

However, there are several "traps".

The voltage measured by the diode probe will be the peak voltage rather than the RMS voltage needed fro the avove calculation.
Vrms = Vpeak/1.414, for a sine wave.

The impedance of your dipole is almost certainly not what you expect!
Try using a dummy load.

The antenna is at best 1 metre away from your multimeter, (you state that the transmitter connects to the antenna via 1 metre of coax).
RF from the antenna will get into you multimeter, especially if it is digital, and cause it to read any old rubbish, which is what is happening to you.
This is a very good example of poor EMC, (Electro Magnetic Compatibility).
Moving the antenna away from your tx and meter, or using a dummy load should clear this situation.

As Nigel said, I hope you have an amateur radio licence for this lot.

Jim GM3ZMA
 
Is the 'diode capacitor rectifier' that you describe intended to be an RF probe? If so, it seems as though it might not be working. As Nigel points out the readings just don't make sense.

If you are licensed for actual operation I'd recommend the following:
A. Do all of your initial testing into a proper dummy load - not a light bulb or other device that will radiate RF. If you are not satisfied with the operation do not connect it to an antenna - it won't work any better and you could cause problems for others.

B. A 1 meter length of coax would put the antenna too close to you or your instruments. It's likely that the antenna isn't behaving predictably because of proximity to other things. RF could also affect your meter and the transmitter itself. Get the antenna up and away from other things, people, etc.

C. Connect the antenna with coax that is one half electrical wavelength long at the frequency of interest. This will repeat the antenna feedpoint impedance -unlikely to be 50 ohms but odds are that this will get you closer to 50 ohms than any other length.

I made the presumption that the transmitter you built is solid state using relatively low voltages. It's possible that you built a vacuum tube transmitter with a power supply that is running at higher voltages. If that is the case you need to make the assumption that the full power supply voltage can appear on the antenna even if the output is connected via capacitor. Capacitors do short circuit occasionally. This further adds to the necessity to have the antenna away from people.

Back to licensing - many of us would discourage operating a transmitter in a way that does not comply with the regulations in your country/state/city. I do realize that many licensing courses cover the basics but don't give you enough information to actually accomlish the tasks necessary for good operation.
 
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