Antenna feed affecting antenna length?

JimmyAlz

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.

Averagesupernova

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.

JimmyAlz

I see! Thank you for your reply :)

YesIam

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

JimmyAlz

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.