2.5 gig antenna

I am wondering if it is possible to lengthen a 2.5 gig antenna such as in a wireless phone? Would running the extra coax change the characteristics of the antenna?
Thank You


  • 8 Comments sorted by Votes Date Added
  • edited July 2005 Posts: 0Vote Up0Vote Down
    You might find that the extra length of coax will be too lossy.
    So any benefits of running coax to an area of better reception would be lost.

    Just an example (this isn't the cable you would use)....
    But RG58 (common 50ohm cable) has a loss of 2.5dB/100ft @ 30Mhz. But has a loss of 16.5dB/100ft @ 900Mhz....so as you can see, this wouldn't be the type of cable you'd use.
    You can use LMR-400 Cable, which has a loss of 6.6dB/100ft @ 2400Mhz. This doesn't seem too bad (relatively speaking of course)....however, just not that for every 3dB loss, that's a reduction in your output power by HALF!! So, if you had 10Watts going out (you'd have less than 1watt coming out of a digital cell phone) with a 6.6dB loss: the first 3dB would drop you to 5Watts...then the 2nd 3dB would drop you to 2.5Watts then take off the 0.6dB for approx 2watts left (didn't do the math!).

    I think that cabling type is semi-rigid (like copper piping).

    I've heart of people using wire wrappings and copper hangers to do something similar....but I question the loss vs. reception benefits.
    You might want to look in to a 2.5gig amp that you could then put an antenna to.

    Now, all that being said about cable losses, there are antennas with gain....so if you can get enough signal to these types of antennas (which would probably be a powered antenna) then you wouldn't have a problem I'd suppose!


    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • edited July 2005 Posts: 0Vote Up0Vote Down
    Another issue to take into consideration beyond the loss issue is that antennas are designed to be a specific length. This length is typically 1, 3/4, 5/8, 1/2 or 1/4 the wavelength of the signal. This is because an antenna acts like a pipe on a church organ, it is designed to resonate (amplify) a specific frequency. Just like trying to make an organ pipe designed to play an A# will not sound very loud if you try to force it to play a G note, an antenna designed for 900MHz will not work very well for a 2.45 GHz signal. The full wavelength for 2.45 GHz is 4.8 inches, further running down the list of standard sizes listed above produces 3.6, 3.0, 2.4 and 1.2 inches. A cell phone antenna will likely be using the 1/4 wavelength due to length restictions, I wouldn't consider replacing it with any antenna other than 4.8, 3.6, 3.0 or 2.4 inches in length. The are many sites on the net which explain the pros and cons in choosing which fractional wavelength antenna to use. Impedance matching is also an issue to take into consideration.

    ·Everyone can see the beauty in a rose, not everyone can see the beauty in a cockroach.

    Post Edited (Paul Baker) : 7/12/2005 1:56:17 PM GMT
  • edited July 2005 Posts: 0Vote Up0Vote Down
    Hi all;

    Finally a thread I can speak with some authority on. Gain of the antenna is a consideration. Can you be more specific with you application. Having an antenna at resonance (wavelength) is paramount. If you are off resonant frequency you can exceed SWR (standing wave ratio). In simpler terms you can have a portion of the energy intended to go out of the antenna actually travel back down to the transmitting device. This can cause excess heat in·certain components, hence destroying them.

    Coax designed for high frequecnies is very expensive due to minimizing skin effect at GHZ frequencies. You can pay as much as 10 bucks for just a couple of feet and still have 1 to 1.5 DB loss at 10 feet.

    Also just because a 1/4 wavelength antenna is used does not mean it has not been "matched" to the transmitter. If you decide to go full wavelength ,which in theory will give you better gain, you wil actually lose power due to the mismatched antenna. Matching the antenna to wavelength does not "amplify", it however maximizes the rf emitted. Energy cannot be created or destroyed, just changed in form. Using an antenna designed for 900 MHZ to transmit 2.5 GHZ will destroy the transmitter eventually.

    Perhaps a cell phone tech type can let us know if matching devices are used in cell phones. If you need help with directional antennas at high frequencies I would be glad to assist you. I'm a real geek when it comes to RF (Radio Frequency). Remember "The geeks shall inherit the earth"

    BTW Steve B nice explantion of "Gain". I struggle to simplify things. that was well done.

  • edited July 2005 Posts: 0Vote Up0Vote Down
    Thanks Sofalogic.!
    I have these moments of clarity....it's these times that I have to get as much done as I can...cuz at any moment I'll get my head stuck in my ar$e and will be back to not having a clue! haha
    I loved RF class in college and have my Basic/Advanced Ham license (no Morse endorsement...ears can't stay tuned long enough haha). OH, I'm in Canada, we still have morse up here!

    I had neglected to mention the matching of transmission lines (your coax) to antennae. You can certainly get away with being slightly off (SWR of less than 2) depending on how much power you are sending out. I can't remember the formula, but there's a ratio wrt SWR explaining how much power will actually be reflected back to your transmitter.

    As far as cell phones, they old analogue ones certainly had to be matched. USually some sort of Matching cct or stub traces on a PCB. Unsure what they do with digital cells (as I'm sure it's more important for proper transmissions).

    If you've ever set up a CB in a vehicle, following the instructions (yes, I did this at one time haha) they recommend buying a little SWR meter from radioshack and running this inline. With this set up you had to rough measure the length of your antenna (dipole)--leaving it a bit long--and snip bits of it off until your SWR meter got down as close to 1 as possible (1 is theoretical...not practical).
    Dipoles are basically your wire sticks! (like your car radio antenna) There are many other different types of antennae, all of which are "tuned" a different way. Some, you can snip elements to tune them....other you add inductors/capacitors to match them up.
    There's some complicated math involved with doing this....but thankfully some wise ppl have done some things with java/flash so that you just enter what you want and it comes back and tells you the lengths you need.

    Our Radar runs at 5.6Ghz and runs something called waveguide. It's basically square copper tubing who's dimensions reflect the wavelength of our transmitter (which is about 5cm). So I'd assume (without calculating) that you're wavelength would be somewhere around 12-13cm. This is small enough on it's own (~3inches) so building one at quarter wavelength becomes the work of elves!
    At this freq, you'd be looking at a groundplane PCB type of antenna (basically a circuit board who's back was the ground plan and a funky microstrip trace on the topside that would be your 'active' antenna element.

    I don't know if inductive pickups will cause any issue with your transmitter (as they aren't directly connected---a coil near your phones antenna would pick up the signal and send it on to an antenna (or amplifier first)...).... Those in-car installs use these types of setups (as most phones don't have a direct antenna connector).

    Let us know what you do!

    Just went back to read my post....SORRY FOR BEING WORDY FOLKS!!· Too much salsa at lunch I guess!



    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • edited July 2005 Posts: 0Vote Up0Vote Down
    Sofalogic said...

    Matching the antenna to wavelength does not "amplify", it however maximizes the rf emitted.

    Sorry I misspoke, I·am aware that wavelength matching does not amplify the signal, I just used a poorly chosen word to describe it. I should have stated that it behaves like a comb·filter, transmitting frequencies near it's·peak frequencies and attenuating frequencies near the notches. But the analogy doesn't really clarify anything unless you understand filters, something that isn't commonly known.

    ·Everyone can see the beauty in a rose, not everyone can see the beauty in a cockroach.
  • edited July 2005 Posts: 0Vote Up0Vote Down
    Hi again all;

    I do apologize if I sounded harsh over the "amplify" comment, It was not meant to be harsh. I almost made a comment on that thought. As I said·I cannot always simplify the answer. Filters are another magic specialty. Rf can be so finicky.

    I did not know anyone was still using waveguide. This must be high power stuff. I have done some 10GHZ antenna work with teflon PCB. I actually experimented with some Yagu-uda designs etched out in miniature form. Lots of fun.

    Have we figured out what the project is for yet??

    Andrew (Sofa)

    Oops! had to edit. I do morse code also now and then. Love the QRP stuff. Low power and a good antenna. Nice challenge. I too am advanced class. I am not going to extra class·just to prove I could do 13 WPM at one time.

    Post Edited (Sofalogic) : 7/13/2005 2:37:41 AM GMT
  • edited July 2005 Posts: 0Vote Up0Vote Down
    Hi Andrew,

    There are still quite a few places that still use waveguide of one sort or another.
    All Canadian weather radars use it as well as all the NOAA NExRad sites.
    The Canadian Pk output power is typically about 190kW but due to our pulse widths and Pulse Rep Frequencies we typically run about 125W average power.
    I got a chance to visit a nexRad site. They run S-band (10cm wavelength) so everything is twice the size of our stuff up here in Canada (we're only 5cm)....and I remember that NexRad sites transmit something on the order of 1megawatt pk power. I don't recall their average power....but do recall that they use klystrons and I thought I heard something about a Traveling Wave Tube....but I didn't think the site I was at was using it. We are just using a Magnetron....nice and compact compared to the Klystrons!

    For our Waveguide, we only have about a 1.5dB loss over 200ft which includes a number of bends and two rotary joints.....so we are quite happy with that and our VSWR sits about 1.1 so that's great!
    Anyhow, I can go on and on about it all....just a nice tidbit....we just converted to dual polarization last year (University of North Dakota beat us to it by a couple months) and we use an antenna mounted receiver to minimize the receive path loss (so that initial WG run loss is only in the transmit path).

    haha....i sound like a proud father yodeling about his baby boy!!



    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
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