Antenna feed affecting antenna length?

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    Antenna Length
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Discussion Overview

The discussion revolves around the impact of antenna feed length on the effective length of antennas, particularly in the context of RF design for a GSM communications module operating at approximately 900 MHz. Participants explore the relationship between feed length, impedance, and antenna performance, considering both theoretical and practical aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant suggests that the length of the antenna feed should not affect the antenna length, as antennas are designed to be resonant at a specific frequency regardless of feed length.
  • Another participant argues that the effect of feed length depends on the load; with a resistive load or a resonant antenna matching the feed impedance, the length of the feed line matters primarily in terms of loss.
  • A different participant expresses uncertainty, indicating that there are both 'yes' and 'no' answers to the question, emphasizing the importance of context such as the type of antenna and whether it is used for transmission, reception, or both.
  • One participant mentions that practical experimentation may yield results that differ from theoretical calculations, suggesting that various factors like grounding and feed line bending can influence performance.
  • A later reply clarifies the specific application involving a GSM module and a quarter-wave antenna, noting that the feed should ideally be close to 50 Ohms for optimal performance.

Areas of Agreement / Disagreement

Participants express differing views on the influence of feed length on antenna performance, with no clear consensus reached. Some emphasize theoretical considerations while others advocate for practical experimentation.

Contextual Notes

Participants acknowledge that the discussion involves assumptions about load types, impedance matching, and the specific application of antennas in GSM communications, which may affect the conclusions drawn.

JimmyAlz
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Does the length of an antenna feed affect the length of the antenna one should use?
Intuitively I'm thinking 'NO' since in some applications the user supplies (variable length) antenna feed cables, but the antenna is still made to be resonant to the application frequency.

BUT in the RF design package I use, the "electrical length" of feeds seem important, and is
always calculated.

I'm just looking to be a little more sure about this before I push the button and produce 100 PCB's.
 
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It depends on the load. With a resistive load or resonant antenna that has the same feed impedance as the lines impedance it makes no difference what the length of the line is other than more length = more loss.
 
I see! Thank you for your reply :)
 
Hi - Um...to this question, for what it's worth, as far as I know, I can give you a definite 'yes' and 'no'. Not trying to be funny...I assume you are talking about a long wire antenna? I have made long wires, Yagis, folded dipoles and others over the years. My first thought is just how close do you want to be...are you talking about a receiving only or both transmission and receiving. Either way everything in life is a compromise. So...I would say you can start out without any particular length of feed line and depending on loads and any matching networks go from there. I again assume you are concerned about SWR. Here is where theory and experimentation comes in...and I will go out a limb and say that I would be very surprised that any amount of calculation would get one as close as the acutual experimentation. Grounding. bending the feed lines, looking at things cross eyed and any number of things will give different results. In my humble opinion, I would say to make your feed line any approprate length just to get from here to there. I don't think that you will have to worry about producing any '100 PCB's'. You can play with your Smith Charts and if you can't get your SWR to 1:1, I would just say hit m' with what you got. I would like to hear any other slant on this.
WH6OC
 
YesIsam: Thank you for your input :)
I see I should have been a little more specific:

I am working with a GSM communications module, to be mounted on a printed circuit board.
GSM = European cellular phone standard.
So working frequency is about 900Mhz.
And yes it will both receive and transmit.
I have chosen a quarter-wave antenna (83mm) because any longer and it would not fit the case of the product.

I agree with you that experimentation/prototyping is always
the key to any successful design.

Anyway after Averagesupernova's answer I have decided I'm in the green, as long as I make the feed as close to 50 Ohm as I can get.
 

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