Antennas 101 & 102 articles

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I didn't read the whole thing yet but looks promising.

Maybe we could get them to let us re-print it in the new on-line mag. This information looks like it could be a keeper to me?

Thanks Heather for sharing.
 

helipaddy

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And make sure your length of Co-Ax cable is a multiple of the Wavelength of the Frequency of the Antenna
 

kc0iv

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And make sure your length of Co-Ax cable is a multiple of the Wavelength of the Frequency of the Antenna
If the antenna is properly tuned to the coax the length of the coax doesn't make a difference. Any excessive length is only a loss.

The exception to this is when you are tuning an antenna then the coax should be 1/2 wavelength electrically. In this case the meter will show the true antenna impedance.

Leon
(kc0iv)
 

ckurz7000

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If the antenna is properly tuned to the coax the length of the coax doesn't make a difference. Any excessive length is only a loss.

The exception to this is when you are tuning an antenna then the coax should be 1/2 wavelength electrically. In this case the meter will show the true antenna impedance.

Leon
(kc0iv)
I second that. -- Chris.
 

helipaddy

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I stand Corrected!
I need to delve into this a bit more
Paddy
 

helipaddy

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I have to get this right in my head

Quote:

If the antenna is properly tuned to the coax the length of the coax doesn't make a difference. Any excessive length is only a loss.

There seems to be a bit of a contradiction in this, you say Co-ax length doesnt make a difference, then you say an excessive length makes a loss, so there must be an optimum length?

The object of this is to match the impedance of the transmitter and antenna system to ensure as little loss as possible.

But the transciever does not see the antenna alone. the Transciever is reacting to the entire antenna system, including the coax. Coax not only has resistance( which changes with coax length, but it also has capacitance which also changes with different coax length. Resistance (Impedance), determines how the transmission line (coax)and antenna match the radio's requirement to send the greatest amount of signal to the antenna. But capacitance controls which frequency the antenna is best tuned at. Changing the capacitance of the antenna system, will alter the frequency at which your radio sees the best SWR.

Where am I goin wrong?
 

kc0iv

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I have to get this right in my head

Quote:

If the antenna is properly tuned to the coax the length of the coax doesn't make a difference. Any excessive length is only a loss.

There seems to be a bit of a contradiction in this, you say Co-ax length doesnt make a difference, then you say an excessive length makes a loss, so there must be an optimum length?

The object of this is to match the impedance of the transmitter and antenna system to ensure as little loss as possible.

But the transciever does not see the antenna alone. the Transciever is reacting to the entire antenna system, including the coax. Coax not only has resistance( which changes with coax length, but it also has capacitance which also changes with different coax length. Resistance (Impedance), determines how the transmission line (coax)and antenna match the radio's requirement to send the greatest amount of signal to the antenna. But capacitance controls which frequency the antenna is best tuned at. Changing the capacitance of the antenna system, will alter the frequency at which your radio sees the best SWR.

Where am I goin wrong?

I agree Padraic the object is to match the transmitter and antenna with the least loss possible.

First let me give you an example. In this example I won't take the time to calculate the electrical length of the coax.

If you were connecting a transceiver to the antenna and the physical coax length was 10 feet. The electrical 1/2 wave length is determined to be 3 feet. As you can see you would have to use 4 lengths of coax to reach from the transceiver to the antenna if you followed the concept of using 1/2 half wave lengths multiplies. Which would add additional loss to the system.

There are three part of the system ( transceiver, antenna, and coax ) if all three are all the same impedance the coax length only needs to be the physical distance needed to go from the transceiver to the antenna. Any extra length would only add additional loss to the system.

This might not be a problem on VHF other than what to do with the extra coax, but at low frequencies you could be dealing with several hundred feet of excessive coax.

Looking at your concerns.

Coax has all three components resistance, inductive, and capacitive reactant. The inductive and capacitive reactants cancel leaving only the resistance.

If the antenna is tuned to the correct frequency it will show only this resistance. There won't be a reactants.

The antenna length should be adjusted so that the correct length is used for the frequency it question. This is why you adjust the length in the final location the will be mounted.

Looking at a real antenna for a moment. In most aircraft they use a 1/4 wavelength mono-pole antenna. When the length is properly set it's resistance will be 30 ohms NOT 50 ohms like most people think. This is why you will see many of the antennas mounted at about a 45 degree angle relative to the aircraft skin. This raises it impedance to the normal 50 ohms used by the system.

Many of the commercial antennas introduce a capacitive reactant so when properly tuned to length it will have a resistance of 50 ohms. Normally, these antennas come with an length longer than needed and the installer cuts then to length to match the final frequency.

One other way a 1/4 wavelength mono-pole antenna can be adjusted is to place a 1/4 wave 72 ohm coax. This will transform the 30 ohm impedance to a 50 ohm impedance.

Coax length should never be used to match the antenna to the transceiver other than the one example I just gave.

Antennas are more involved than most people think if they are trying to get the best signal possible signal from the system. Thankfully, most people are not trying to get that perfect 1:1 VSWR and can accept something less than perfect.

I hope I have answer some of your concerns. If not then just say so and I'll try to go into greater details.

Leon
(kc0iv)
 

RockyMeLad

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Excellent description Leon.

As an aside (does not change anything Leon said), I use a full half-wave dipole, rather than a quarter-wave (half a dipole with a ground plane). My dipole would have about 73 ohms impedance if straight, but by bending it some in the middle (at the feed point) I lower the impedance to about 50 ohms to get the desired "impedance match" between the coax and the antenna. Don't be confused, it's all in the angles as to whether bending "increases" or "decreases" the impedance.

There were numerous "old wives tales" concerning RF from the early days of CB radio. One that just won't die is the "tune the antenna by changing the length of the coax". It "sort of" works at/or extremely close to one specific frequency. Our aviation radios need to be able to tune across a range of frequencies. A properly tuned antenna with good impedance matching will do this easily.
 
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