Antenna feed affecting antenna length?

In summary, 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.
  • #1
JimmyAlz
26
0
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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  • #2
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.
 
  • #3
I see! Thank you for your reply :)
 
  • #4
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
 
  • #5
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.
 

1. How does the antenna feed affect antenna length?

The antenna feed is the connection between the antenna and the transmitter or receiver. It affects antenna length by determining the impedance of the antenna, which is a measure of how easily it can send and receive radio waves. A mismatched impedance can cause reflections and reduce the efficiency of the antenna, resulting in a shorter effective length.

2. What is the ideal impedance for an antenna feed?

The ideal impedance for an antenna feed depends on the type of antenna and the frequency of operation. Generally, a 50-ohm impedance is considered to be the most efficient and commonly used for antennas in the radio frequency range.

3. Can the length of the antenna feed affect antenna performance?

Yes, the length of the antenna feed can affect antenna performance. A longer feed can introduce additional losses and affect the impedance match, resulting in a decrease in antenna efficiency. It is important to keep the feed length as short as possible to minimize these effects.

4. What are some ways to optimize the antenna feed length?

One way to optimize the antenna feed length is to use a matching network, such as a balun or a matching transformer, to adjust the impedance of the antenna and minimize reflections. Another way is to use a feed line with low loss and a high-quality connector to ensure efficient power transfer.

5. How does the location of the antenna feed affect antenna length?

The location of the antenna feed can affect antenna length by changing the radiation pattern of the antenna. Placing the feed at the center of the antenna will result in a symmetric radiation pattern, while placing it off-center can introduce asymmetries and alter the effective length of the antenna.

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