Delta Prime
- Jul 29, 2020
- 2,987
- Joined
- Jul 29, 2020
- Messages
- 2,987
TV reception antennas are nominally designed for either 75-ohm unbalanced or 300-ohm balanced standard impedances.
An antenna’s impedance plot demonstrates how its input resistance and reactance vary wildly across different frequencies. While exact numbers vary by element length, all standard television reception antennas (like Yagi-Uda and log-periodic designs) follow the same universal impedance.
Because commercial impedance curves (Smith charts or VSWR plots) are proprietary, they are not typically published.
www.rohde-schwarz.com
An antenna’s impedance plot demonstrates how its input resistance and reactance vary wildly across different frequencies. While exact numbers vary by element length, all standard television reception antennas (like Yagi-Uda and log-periodic designs) follow the same universal impedance.
Because commercial impedance curves (Smith charts or VSWR plots) are proprietary, they are not typically published.
Understanding the Smith chart
The Smith chart is a key tool in RF engineering and circuit design. Learn Smith chart basics, how to read a Smith chart and Smith chart impedance matching.
www.rohde-schwarz.com
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1) The impedance of an antenna isnt directly related to frequency. So no, you wont find a freq. Vs imp. plot
2) The impedance will be quite constant across a wide range of frequencies
3) The impedance is determined by the type/style of the feedpoint and overall antenna, regardless of frequency
e.g. for #3)
the feedpoint impedance of a plain dipole is typically 50 to 75 Ohms
the feedpoint impedance of a folded dipole is typically 300 Ohms
Usual feedpoint impedances of a dipole on its own.
Once additional elements are added, this effectively lowers the feedpoint impedance.
A plain dipole can drop down to around 25 to 50 Ohms
A folded dipole can drop down to around 200 Ohms
Dave
cheers
Dave
And what is f here ? :
So changing a frequency does not change the impedance ? What do you mean by 'quite' ?
---> And why do antenna tuners exist for ?
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So changing a frequency does not change the impedance ? What do you mean by 'quite' ?
---> And why do antenna tuners exist for ?
not the meaning I was using for quite .... language difference between you and me
change quite to reasonable range
The freq range of an antenna has to move significantly ( depending on construction and other factors)
before the impedance changes significantly
Theory is a good guide, practical operation/testing/using is where you really find out what works and
what doesnt
This is more noticeable on HF than on VHF and up into microwave freq's.
Antenna tuners are needed to keep the
1) transmitter matched to the antenna
2) the variation of type of antenna
3) resonant freq of an antenna
Your response hinted at that maybe you dont quite understand antenna resonance ?
I gotta leave home for work .... will finish this at work
OK back .....
Amateur radio operators often use antenna tuners on HF antennas 3 - 30 MHz range
because they want the transmitter to see a good load across a very wide range
But what most dont understand is that, tho the transmitter is happy looking at a good load,
The antenna is horribly inefficient anywhere it's used away from its resonant frequency.
They also love to use a single, random length, long wire antenna, resonant nowhere on any
of the bands of interest, so again, they use a tuner to make some use of it
Antenna construction also affects the resonant bandwidth and therefore the impedance.
The thinner the antenna element, the narrower the bandwidth and therefore a tuner is
needed even to move the transmitter as little as 200kc.
Conversely, at say the 2m band, 144 - 148MHz, or even 70cm band 430 - 450MHz,
the resonance and impedance of the antenna can stay reasonably consistant over many MHz.
Even at 23cm band 1240 - 1300MHz, 60MHz of bandwidth, an antenna can still be resonant
across that 60MHz and therefore the impedance doesnt vary much ... + - a few Ohms.
This may sound bad in theory, but in practice is irrelevant.
So you can see there are a number of factors that allow the impedance to stay reasonably
constant over a wide range.
As an amateur radio operator and also for professional work, I have been playing with antennas
for the best part of 50 years
cheers
Dave
VK2TDN
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And what will you do if you have that info?
I would think the antenna gain vs. frequency (into a fixed load) would be of more interest.