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Is there a way to estimate how long an antenna is? Answered

If I built a radio transmitter and then attached it to an antenna of unknown length, is there a way to calculate the length of it using transmissions? Or, as a separate question, if I had an antenna of known length and then increased or decreased the length, would there be a way to estimate the new length from transmission properties (basically without physically measuring the antenna? Assume the antenna is a straight line.

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15 Replies

rickharris (author)2017-06-11

Why would your antenna length be unknown?

Generally accepted that a full wave length is the best match, after that 1/2 and 1/4 follow. Often 1/4 wave length is chosen as a practical length.

We are all assuming your talking about relatively low frequency - if you get into microwave lengths practically anything transmitts even little tags of solder.

kakashibatosi (author)2017-06-11

It wouldn't be, but I figured the condition would act as a proof of concept. Lets say your antenna started at 12 inches in length, and it was cut repeatedly until it reached say 1 inch in length. Would there be a way to measure the changes in length in between?

steveastrouk (author)2017-06-11

At one inch, you ARE deeply in microwave area - around 3 gig if we assume its 1/4 wave antenna.

rickharris (author)2017-06-11

Calculate the 1/4 wavelength of your antenna. Cut it slightly long ( a few inches) then get a SWR meter and cut to minimise the SWR.

WHat freq are we dealing with?

Your question "Would there be a way to measure the changes in length in between?" doesn't make sense

kakashibatosi (author)2017-06-11

I suppose I would be in the microwave region if my starting point was an antenna 150mm long. I was just curious if there was a way of determining how long a wire was after I cut it using only radio. Like "here's a 6 inch wire. If I cut off part of the end, could you calculate how long it is now?"

rickharris (author)2017-06-11

1. Once you entre microwave regions your in a mine field where very special equipment is needed to do more than just guess.

2. When Antennas are formed they are generally made using the formulae you already have, then they are field tested to check coverage range etc, there isn't any way to discover this other than by transmitting through the antenna and riding around with a field meter checking the field strength. This maps the coverage locus. this will be the same if the antenna is used to transmit or receive.

3. If you intend to do this practically, and do not have the underlying theory/understanding you not very likely to be very successful. you may well be no better off than taking a best guess, cutting to length and crossing your fingers.

rickharris (author)2017-06-12

In the 3 cm to 10 cm radar range your looking at wave guide as a normal wire antenna would be too leaky at any reasonable power level.

Over a small range I guess the Standing Wave Ratio will have some relationship to the length of the antenna.

Practically your barking up the wrong tree here.

steveastrouk (author)2017-06-10

Yes, you can observe the "SWR", which is, without adding a major essay, a measure of how much energy bounces out of the antenna, Get the SWR wrong, and if you have a high power transmitter, you can blow it up.

You need a rough handle on the frequency the transmitter is expected to work at. Unless you are in the Megahertz, the antenna has to be many metres long, since it needs to be 1/2 or 1/4 of the wavelength v=n lambda, Lambda(wavelength)= c (speed of light)/frequency.

iceng (author)2017-06-10

+1

rickharris (author)2017-06-11

Yes assuming you have a suitable SWR meter you can cut for the lowest SWR reading. In the RAF in the 60's/70's this was how we tuned fixed length mobile antenna to their assigned frequency.

HOWEVER you only know you have gone too far when the reading starts to go up again! so it is somewhat a skill.

kakashibatosi (author)2017-06-11

Assuming that you started with a 12 inch antenna, and were aiming for 1 inch, could this be tracked via this method? Is it too much of a length difference? Maybe more appropriate starting at 6 inches and approaching 1?

kakashibatosi (author)2017-06-10

So if i made an antenna from copper wire, could i see the difference if I cut segments off the wire while measuring the "SWR"?

steveastrouk (author)2017-06-11

You'll get a "good" SWR at other, less optimal lengths of antenna too. 1/4 wave and 1/2 wave are the simplest dipoles.

iceng (author)2017-06-10

Can you actually ..... do both at the same time ?

rickharris (author)2017-06-11

As a Radar eng in the RAF, on several units we checked Standing Wave Ratio by dropping a line of neon filled glass tubes into the wave guides narrow dimension through a slot.

The neon would light showing the standing wave as a graph - Neat! Tune a stub to get minimum SWR.

The theory as to why the RF didn't leak out of the slot used to make my head hurt.