Cable impedance?

Archived from groups: alt.internet.wireless (More info?)

Hi,

I'm in the process of making some indoor antennas for my home network. I
have a few different designs etc, but have so far been unable to locate
an impedance for the cable I should use...

is it just 75ohm co-ax like for TV? or something different?

I have a Billion 7500G switch/router/AP and MiniTar PCI cards (came as a
package)..

Any help much appreciated.

Thanks,

Pete
9 answers Last reply
More about cable impedance
  1. Archived from groups: alt.internet.wireless (More info?)

    In article <41bfe14b@news.eftel.com>,
    Filthy Pierre <petermc_nospam@andersenit.com.au.nospam> wrote:
    :I'm in the process of making some indoor antennas for my home network. I
    :have a few different designs etc, but have so far been unable to locate
    :an impedance for the cable I should use...

    :is it just 75ohm co-ax like for TV? or something different?

    No, don't use that co-ax, it won't work!

    A google search on wifi antenna impedance shows that most
    models are 50 ohm.

    See for example http://helix.remco.tk/
    --
    If a troll and a half can hook a reader and a half in a posting and a half,
    how many readers can six trolls hook in six postings?
  2. Archived from groups: alt.internet.wireless (More info?)

    On 15 Dec 2004 08:13:33 GMT, roberson@ibd.nrc-cnrc.gc.ca (Walter
    Roberson) wrote:

    >In article <41bfe14b@news.eftel.com>,
    >Filthy Pierre <petermc_nospam@andersenit.com.au.nospam> wrote:
    >:I'm in the process of making some indoor antennas for my home network. I
    >:have a few different designs etc, but have so far been unable to locate
    >:an impedance for the cable I should use...
    >
    >:is it just 75ohm co-ax like for TV? or something different?

    >No, don't use that co-ax, it won't work!
    >A google search on wifi antenna impedance shows that most
    >models are 50 ohm.
    >See for example http://helix.remco.tk/

    Well, here I get to challenge the traditional orthodoxy. Any book on
    system design will declare that it's a really good idea to match all
    the impedances. If the antenna is 50 ohms, and the access point radio
    is 50 ohms, it would seem to be proper to use 50 ohm coaxial cable.

    However, if one uses 75 ohm cable, it's not as detrimental as it would
    seem at first glance. The 50 to 75 ohm mismatch is a VSWR of 1.5:1
    which generates a loss of 4% of the power or 0.18dB.
    http://www.csgnetwork.com/vswrlosscalc.html
    http://www.microwaves101.com/encyclopedia/calvswr.cfm
    If that's tolerable, then 75 ohm coax should work. There will also be
    some detrimental effects to the the antenna pattern and frequency
    response if the antenna is fairly high Q (narrow band). However, this
    is normally not a problem with most (not all) 2.4GHz antennas.
    Similarly, the ceramic input filters in the access point might not
    like the mismatch. I haven't seen any problems, but I also haven't
    done any proper testing.

    Incidentally, most 2.4GHz antennas are nowhere near 50 ohms across the
    entire 83.5Mhz band. For example, see:
    http://www.LearnByDestroying.com/pics/antennas/coffee2400/slides/vswr-01.html
    and note that the VSWR of the common coffee can antenna is 2:1 (either
    25 or 100 ohms) near the band edges (2400/2450). Not exactly a
    perfect match but good enough as a 2:1 mismatch is "only" a 0.5dB
    loss. My guess(tm) is that either 50 or 75 ohm coax would work
    equally well with such an antenna.

    I have a fairly good supply of 75 ohm surplus CATV coax and have used
    it in a few installations where price was more important than
    performance. I've also designed and tested a few 75 ohm antennas. My
    50 ohm test equipment has input/output pads to deal with 75 ohm
    systems.

    A big problem is proper connectors. The common N, BNC, TNC, SMA
    connectors are available in both 50 and 75 ohm diameters. However,
    the more exotic R-TNC and R-SMA found on access points are not
    available. I have to use coax adapters (or butchery) at the access
    point and switch to standard connectors.

    As for the selection of coax cables, don't just grab a chunk of RG-6/u
    or RG-11/u and start using it at 2.4GHz. These have turned into a
    family of cables with radically different characteristics. Even the
    "satellite grade" of TV coax is only rated to 2GHz and is horribly
    lossy at 2.4GHz. For comparison, LMR-400 is 6.7dB/100ft, while the
    best RG-6/u is approx 11dB/100ft. For short runs, such loss is
    tolerable, but don't try it for long runs.

    Anyway, enough heresy. Use 50 ohm coax if possible, and only use 75
    ohms if you have a good reason to do so.


    --
    Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 AE6KS 831-336-2558
  3. Archived from groups: alt.internet.wireless (More info?)

    The previous post is true for a 75 ohm source feeding into a 50 ohm load,
    but the situation is a little more complicated when a 50 ohm source feeds a
    75 ohm transmission line which terminates with a 50 ohm load.

    In this situation the apparent impedance that the source sees is the load
    impedance phase shifted by
    2 x pi x coax_length / wavelength_in_coax

    The wavelength in coax will be roughly 2/3 of the wavelength in vacuum, (ie
    8cm and 12.5cm respectively).

    If the coax happens to be an integer number of half wavelengths long then
    the impedance as seen by the source will be 50 ohms. This will give a VSWR
    of 1:1.

    If the coax happens to be an odd number of quarter wavelengths long then the
    impedance as seen by the source will be 75*75/50 = 112.5 ohms. This will
    give a reflection coefficient of 0.384, and a VSWR of 2.25. 14.8% of the
    power will be reflected, ie a transmission loss of 0.7dB. This ignores any
    losses due to the coax cable itself which will rise linearly with cable
    length.

    Other coax lengths (ie not an integer number of quarter wavelengths) will
    make the load appear reactive to some degree, with a transmission loss of
    between 0dB and 0.7dB.

    The reasons for using the right impedance cable are:-
    1. There is a no transmission loss due to reflections of power back to
    the source at changes of impedance.

    2. Reflections bouncing up and down the mismatched coax will give rise to
    a time delayed signal interfering with the desired signal. In the above
    scenario (75 ohm cable and 50 ohm load) there will be 14.8% x 14.8% = 2.2%
    of the signal present as time delayed interference, ie about 17dB down on
    the desired signal. This is probably low enough not to be a problem with
    digital signals.

    3. The reflected signal will add to the peak voltage and peak current
    strain on the source. Hopefully the wireless link designers will have
    allowed for an awful mismatch in case of bad siting of the antenna, so this
    should not be a problem.

    Colin


    "Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
    news:3lp0s01pkivqb5nnj3a1mjub2l974qu5d3@4ax.com...
    > On 15 Dec 2004 08:13:33 GMT, roberson@ibd.nrc-cnrc.gc.ca (Walter
    > Roberson) wrote:
    >
    >>In article <41bfe14b@news.eftel.com>,
    >>Filthy Pierre <petermc_nospam@andersenit.com.au.nospam> wrote:
    >>:I'm in the process of making some indoor antennas for my home network. I
    >>:have a few different designs etc, but have so far been unable to locate
    >>:an impedance for the cable I should use...
    >>
    >>:is it just 75ohm co-ax like for TV? or something different?
    >
    >>No, don't use that co-ax, it won't work!
    >>A google search on wifi antenna impedance shows that most
    >>models are 50 ohm.
    >>See for example http://helix.remco.tk/
    >
    > Well, here I get to challenge the traditional orthodoxy. Any book on
    > system design will declare that it's a really good idea to match all
    > the impedances. If the antenna is 50 ohms, and the access point radio
    > is 50 ohms, it would seem to be proper to use 50 ohm coaxial cable.
    >
    > However, if one uses 75 ohm cable, it's not as detrimental as it would
    > seem at first glance. The 50 to 75 ohm mismatch is a VSWR of 1.5:1
    > which generates a loss of 4% of the power or 0.18dB.
    > http://www.csgnetwork.com/vswrlosscalc.html
    > http://www.microwaves101.com/encyclopedia/calvswr.cfm
    > If that's tolerable, then 75 ohm coax should work. There will also be
    > some detrimental effects to the the antenna pattern and frequency
    > response if the antenna is fairly high Q (narrow band). However, this
    > is normally not a problem with most (not all) 2.4GHz antennas.
    > Similarly, the ceramic input filters in the access point might not
    > like the mismatch. I haven't seen any problems, but I also haven't
    > done any proper testing.
    >
    > Incidentally, most 2.4GHz antennas are nowhere near 50 ohms across the
    > entire 83.5Mhz band. For example, see:
    > http://www.LearnByDestroying.com/pics/antennas/coffee2400/slides/vswr-01.html
    > and note that the VSWR of the common coffee can antenna is 2:1 (either
    > 25 or 100 ohms) near the band edges (2400/2450). Not exactly a
    > perfect match but good enough as a 2:1 mismatch is "only" a 0.5dB
    > loss. My guess(tm) is that either 50 or 75 ohm coax would work
    > equally well with such an antenna.
    >
    > I have a fairly good supply of 75 ohm surplus CATV coax and have used
    > it in a few installations where price was more important than
    > performance. I've also designed and tested a few 75 ohm antennas. My
    > 50 ohm test equipment has input/output pads to deal with 75 ohm
    > systems.
    >
    > A big problem is proper connectors. The common N, BNC, TNC, SMA
    > connectors are available in both 50 and 75 ohm diameters. However,
    > the more exotic R-TNC and R-SMA found on access points are not
    > available. I have to use coax adapters (or butchery) at the access
    > point and switch to standard connectors.
    >
    > As for the selection of coax cables, don't just grab a chunk of RG-6/u
    > or RG-11/u and start using it at 2.4GHz. These have turned into a
    > family of cables with radically different characteristics. Even the
    > "satellite grade" of TV coax is only rated to 2GHz and is horribly
    > lossy at 2.4GHz. For comparison, LMR-400 is 6.7dB/100ft, while the
    > best RG-6/u is approx 11dB/100ft. For short runs, such loss is
    > tolerable, but don't try it for long runs.
    >
    > Anyway, enough heresy. Use 50 ohm coax if possible, and only use 75
    > ohms if you have a good reason to do so.
    >
    >
    > --
    > Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    > 150 Felker St #D http://www.LearnByDestroying.com
    > Santa Cruz CA 95060 AE6KS 831-336-2558
  4. Archived from groups: alt.internet.wireless (More info?)

    On Thu, 16 Dec 2004 20:45:19 -0000, "nospam" <me@privacy.net> wrote:

    >The previous post is true for a 75 ohm source feeding into a 50 ohm load,
    >but the situation is a little more complicated when a 50 ohm source feeds a
    >75 ohm transmission line which terminates with a 50 ohm load.
    >
    >In this situation the apparent impedance that the source sees is the load
    >impedance phase shifted by
    > 2 x pi x coax_length / wavelength_in_coax
    >
    >The wavelength in coax will be roughly 2/3 of the wavelength in vacuum, (ie
    >8cm and 12.5cm respectively).
    >
    >If the coax happens to be an integer number of half wavelengths long then
    >the impedance as seen by the source will be 50 ohms. This will give a VSWR
    >of 1:1.
    >
    >If the coax happens to be an odd number of quarter wavelengths long then the
    >impedance as seen by the source will be 75*75/50 = 112.5 ohms. This will
    >give a reflection coefficient of 0.384, and a VSWR of 2.25. 14.8% of the
    >power will be reflected, ie a transmission loss of 0.7dB. This ignores any
    >losses due to the coax cable itself which will rise linearly with cable
    >length.
    >
    >Other coax lengths (ie not an integer number of quarter wavelengths) will
    >make the load appear reactive to some degree, with a transmission loss of
    >between 0dB and 0.7dB.

    Agreed on all points. However, you're ignoring the effects of coax
    cable loss. Let's say we're playing with a 50ft chunk of CATV coax
    with a loss of about 3dB. That means that only half of the tx power
    makes it from the xmitter to the load, and only half of the reflected
    power makes it back from the load to the source. I just burned an
    hour trying to work out the exact VSWR at the source for a 3dB cable
    loss, and keep getting rediculous answers. I can pull it off a
    nomagram in the ARRL Handbook, but can't seem to get the numerical
    answer to agree. Maybe 8 hours of snooze and some fast reading will
    provide an answer tomorrow.

    >The reasons for using the right impedance cable are:-
    >1. There is a no transmission loss due to reflections of power back to
    >the source at changes of impedance.

    Yeah, but when the mismatch loss is 0.7dB maximum, which is about the
    same as contributed by a typical connector pair and a short pigtail, I
    wouldn't consider that a serious problem. Again, let me point out
    that the antenna VSWR is often 2:1 or worse at band edges, which is a
    far worse loss than the mismatch loss.

    Of course, I could "build" a 61 ohm 1/4 wave coaxial matching section
    to deal with the impedance mismatch. It's broadband enough to work
    over the entire 83.5Mhz. However, if I include connectors, the loss
    of the matching section is about equal to the worst case mismatch
    loss, and is therefore a waste of effort.

    >2. Reflections bouncing up and down the mismatched coax will give rise to
    >a time delayed signal interfering with the desired signal. In the above
    >scenario (75 ohm cable and 50 ohm load) there will be 14.8% x 14.8% = 2.2%
    >of the signal present as time delayed interference, ie about 17dB down on
    >the desired signal. This is probably low enough not to be a problem with
    >digital signals.

    Again, you neglected the cable loss. If we use my convenient 3dB coax
    cable loss figure, the reflected power will be 20dB down at the load
    and 23dB down back at the source (where it can do the most damage).
    It's actually a fairly large signal. Starting with +15dBm tx power,
    -23dB yields a reflected signal of -8dBm which is strong enough to
    clobber any received signal if suffiently delayed. However, the
    reflected signal in my 50ft cable will be delayed approximately:
    2 * 50ft / 1ft/nsec / 0.67 = 44 nanoseconds.
    The "2" is because the signal must go up the coax and back. The 0.67
    is the coax cable velocity factor.

    802.11g OFDM uses an 800 nsec guard interval to allow for reflections,
    bounces, and multipath delayed transmissions that cause intersymbol
    interference or delay spread. 44 nsec out of 800nsec is not a problem
    (at any amplitude).

    >3. The reflected signal will add to the peak voltage and peak current
    >strain on the source. Hopefully the wireless link designers will have
    >allowed for an awful mismatch in case of bad siting of the antenna, so this
    >should not be a problem.

    Strain? At 50 milliwatts, there's no strain on the transmitter
    devices. 802.11b/g require that the transmitting device be linear.
    That give the power device considerable tolerance to reflected power.
    The devices are also made to survive transmitting into open circuits
    and shorts without VSWR senseing and protection. Actually, I think
    the original Prism 1 and 2 cards would detect VSWR:
    http://www.allaboutjake.com/network/linksys/wlanexpert.html
    Yep. However, I don't think they did anything with the numbers to
    protect the xmitter. Incidentally, I've used the above program to
    test and "sweep" antennas. You haven't seen VSWR until you've build a
    coffee can antenna.

    Anyway, I don't recommend using 75ohm CATV coax if 50 ohm coax is
    available. However, it does work if needed.

    HyperLink 2.4Ghz amplifier that uses 75 ohm coax:
    http://www.hyperlinktech.com/web/ha2401f_agc.php
    Yeah, I know it's matched properly to 75 ohms.

    Using 75 ohm hardline for 2.4Ghz:
    http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html


    >>Colin

    >"Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
    >news:3lp0s01pkivqb5nnj3a1mjub2l974qu5d3@4ax.com...
    >> On 15 Dec 2004 08:13:33 GMT, roberson@ibd.nrc-cnrc.gc.ca (Walter
    >> Roberson) wrote:
    >>
    >>>In article <41bfe14b@news.eftel.com>,
    >>>Filthy Pierre <petermc_nospam@andersenit.com.au.nospam> wrote:
    >>>:I'm in the process of making some indoor antennas for my home network. I
    >>>:have a few different designs etc, but have so far been unable to locate
    >>>:an impedance for the cable I should use...
    >>>
    >>>:is it just 75ohm co-ax like for TV? or something different?
    >>
    >>>No, don't use that co-ax, it won't work!
    >>>A google search on wifi antenna impedance shows that most
    >>>models are 50 ohm.
    >>>See for example http://helix.remco.tk/
    >>
    >> Well, here I get to challenge the traditional orthodoxy. Any book on
    >> system design will declare that it's a really good idea to match all
    >> the impedances. If the antenna is 50 ohms, and the access point radio
    >> is 50 ohms, it would seem to be proper to use 50 ohm coaxial cable.
    >>
    >> However, if one uses 75 ohm cable, it's not as detrimental as it would
    >> seem at first glance. The 50 to 75 ohm mismatch is a VSWR of 1.5:1
    >> which generates a loss of 4% of the power or 0.18dB.
    >> http://www.csgnetwork.com/vswrlosscalc.html
    >> http://www.microwaves101.com/encyclopedia/calvswr.cfm
    >> If that's tolerable, then 75 ohm coax should work. There will also be
    >> some detrimental effects to the the antenna pattern and frequency
    >> response if the antenna is fairly high Q (narrow band). However, this
    >> is normally not a problem with most (not all) 2.4GHz antennas.
    >> Similarly, the ceramic input filters in the access point might not
    >> like the mismatch. I haven't seen any problems, but I also haven't
    >> done any proper testing.
    >>
    >> Incidentally, most 2.4GHz antennas are nowhere near 50 ohms across the
    >> entire 83.5Mhz band. For example, see:
    >> http://www.LearnByDestroying.com/pics/antennas/coffee2400/slides/vswr-01.html
    >> and note that the VSWR of the common coffee can antenna is 2:1 (either
    >> 25 or 100 ohms) near the band edges (2400/2450). Not exactly a
    >> perfect match but good enough as a 2:1 mismatch is "only" a 0.5dB
    >> loss. My guess(tm) is that either 50 or 75 ohm coax would work
    >> equally well with such an antenna.
    >>
    >> I have a fairly good supply of 75 ohm surplus CATV coax and have used
    >> it in a few installations where price was more important than
    >> performance. I've also designed and tested a few 75 ohm antennas. My
    >> 50 ohm test equipment has input/output pads to deal with 75 ohm
    >> systems.
    >>
    >> A big problem is proper connectors. The common N, BNC, TNC, SMA
    >> connectors are available in both 50 and 75 ohm diameters. However,
    >> the more exotic R-TNC and R-SMA found on access points are not
    >> available. I have to use coax adapters (or butchery) at the access
    >> point and switch to standard connectors.
    >>
    >> As for the selection of coax cables, don't just grab a chunk of RG-6/u
    >> or RG-11/u and start using it at 2.4GHz. These have turned into a
    >> family of cables with radically different characteristics. Even the
    >> "satellite grade" of TV coax is only rated to 2GHz and is horribly
    >> lossy at 2.4GHz. For comparison, LMR-400 is 6.7dB/100ft, while the
    >> best RG-6/u is approx 11dB/100ft. For short runs, such loss is
    >> tolerable, but don't try it for long runs.
    >>
    >> Anyway, enough heresy. Use 50 ohm coax if possible, and only use 75
    >> ohms if you have a good reason to do so.
    >>
    >>
    >> --
    >> Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    >> 150 Felker St #D http://www.LearnByDestroying.com
    >> Santa Cruz CA 95060 AE6KS 831-336-2558
    >

    --
    Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 AE6KS 831-336-2558
  5. Archived from groups: alt.internet.wireless (More info?)

    Something to think about: Of the two, 50 ohm is optimized for power (at the
    kilowatt level) and 75 ohm is optimized for low loss at high frequencies.
  6. Archived from groups: alt.internet.wireless (More info?)

    On Thu, 16 Dec 2004 23:29:05 -0800, Jeff Liebermann
    <jeffl@comix.santa-cruz.ca.us> wrote:

    >Agreed on all points. However, you're ignoring the effects of coax
    >cable loss. Let's say we're playing with a 50ft chunk of CATV coax
    >with a loss of about 3dB. That means that only half of the tx power
    >makes it from the xmitter to the load, and only half of the reflected
    >power makes it back from the load to the source. I just burned an
    >hour trying to work out the exact VSWR at the source for a 3dB cable
    >loss, and keep getting rediculous answers. I can pull it off a
    >nomagram in the ARRL Handbook, but can't seem to get the numerical
    >answer to agree. Maybe 8 hours of snooze and some fast reading will
    >provide an answer tomorrow.

    relf_coef = (swr - 1) / (swr + 1) = (1.5-1)/(1.5+1) = 0.200
    and
    refl_coef = sqrt(Reflected_power / Forward_power)
    or
    reflected_power/forward_power = 0.200^2 = 0.0400

    However, since my convenient 3dB coax cable loss cuts the forward
    power in half, and the also the reflected power in half, we get:
    (reflected_power/forward_power) = 0.040 / 4 = 0.010
    Working backwards, the reflection coefficient is:
    sqrt(0.010) = 0.100
    which is an input VSWR of about 1.2:1

    Using the same proceedure for the worst case VSWR (odd multiples of
    1/4 wavelength electrical), where the reflection coeficient is 0.384,
    I get a lossless coax VSWR of 2.25:1. However, when I introduce the
    3dB cable loss, the reflection coefficient becomes 0.192 or a VSWR of
    about 1.4:1.

    More crudely, a high loss coax cable improves the input VSWR picture
    by attenuating any reflections. I also use the same idea for running
    10base2 (cheapernet) coax cables using 75 ohm RG-6/u instead of the
    usual RG-58a/u. I works quite well if you only have stations at the
    ends of the cable run (no taps). I've done about 900ft runs without
    the slightest problem. Same idea. The high cable attenuation eats
    the reflections.




    --
    Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 AE6KS 831-336-2558
  7. Archived from groups: alt.internet.wireless (More info?)

    Thanks fellas for the excellent info! I appreciate it!

    Pete

    nospam wrote:
    > The previous post is true for a 75 ohm source feeding into a 50 ohm load,
    > but the situation is a little more complicated when a 50 ohm source feeds a
    > 75 ohm transmission line which terminates with a 50 ohm load.
    >
    > In this situation the apparent impedance that the source sees is the load
    > impedance phase shifted by
    > 2 x pi x coax_length / wavelength_in_coax
    >
    > The wavelength in coax will be roughly 2/3 of the wavelength in vacuum, (ie
    > 8cm and 12.5cm respectively).
    >
    > If the coax happens to be an integer number of half wavelengths long then
    > the impedance as seen by the source will be 50 ohms. This will give a VSWR
    > of 1:1.
    >
    > If the coax happens to be an odd number of quarter wavelengths long then the
    > impedance as seen by the source will be 75*75/50 = 112.5 ohms. This will
    > give a reflection coefficient of 0.384, and a VSWR of 2.25. 14.8% of the
    > power will be reflected, ie a transmission loss of 0.7dB. This ignores any
    > losses due to the coax cable itself which will rise linearly with cable
    > length.
    >
    > Other coax lengths (ie not an integer number of quarter wavelengths) will
    > make the load appear reactive to some degree, with a transmission loss of
    > between 0dB and 0.7dB.
    >
    > The reasons for using the right impedance cable are:-
    > 1. There is a no transmission loss due to reflections of power back to
    > the source at changes of impedance.
    >
    > 2. Reflections bouncing up and down the mismatched coax will give rise to
    > a time delayed signal interfering with the desired signal. In the above
    > scenario (75 ohm cable and 50 ohm load) there will be 14.8% x 14.8% = 2.2%
    > of the signal present as time delayed interference, ie about 17dB down on
    > the desired signal. This is probably low enough not to be a problem with
    > digital signals.
    >
    > 3. The reflected signal will add to the peak voltage and peak current
    > strain on the source. Hopefully the wireless link designers will have
    > allowed for an awful mismatch in case of bad siting of the antenna, so this
    > should not be a problem.
    >
    > Colin
    >
    >
    >
    > "Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
    > news:3lp0s01pkivqb5nnj3a1mjub2l974qu5d3@4ax.com...
    >
    >>On 15 Dec 2004 08:13:33 GMT, roberson@ibd.nrc-cnrc.gc.ca (Walter
    >>Roberson) wrote:
    >>
    >>
    >>>In article <41bfe14b@news.eftel.com>,
    >>>Filthy Pierre <petermc_nospam@andersenit.com.au.nospam> wrote:
    >>>:I'm in the process of making some indoor antennas for my home network. I
    >>>:have a few different designs etc, but have so far been unable to locate
    >>>:an impedance for the cable I should use...
    >>>
    >>>:is it just 75ohm co-ax like for TV? or something different?
    >>
    >>>No, don't use that co-ax, it won't work!
    >>>A google search on wifi antenna impedance shows that most
    >>>models are 50 ohm.
    >>>See for example http://helix.remco.tk/
    >>
    >>Well, here I get to challenge the traditional orthodoxy. Any book on
    >>system design will declare that it's a really good idea to match all
    >>the impedances. If the antenna is 50 ohms, and the access point radio
    >>is 50 ohms, it would seem to be proper to use 50 ohm coaxial cable.
    >>
    >>However, if one uses 75 ohm cable, it's not as detrimental as it would
    >>seem at first glance. The 50 to 75 ohm mismatch is a VSWR of 1.5:1
    >>which generates a loss of 4% of the power or 0.18dB.
    >> http://www.csgnetwork.com/vswrlosscalc.html
    >> http://www.microwaves101.com/encyclopedia/calvswr.cfm
    >>If that's tolerable, then 75 ohm coax should work. There will also be
    >>some detrimental effects to the the antenna pattern and frequency
    >>response if the antenna is fairly high Q (narrow band). However, this
    >>is normally not a problem with most (not all) 2.4GHz antennas.
    >>Similarly, the ceramic input filters in the access point might not
    >>like the mismatch. I haven't seen any problems, but I also haven't
    >>done any proper testing.
    >>
    >>Incidentally, most 2.4GHz antennas are nowhere near 50 ohms across the
    >>entire 83.5Mhz band. For example, see:
    >>http://www.LearnByDestroying.com/pics/antennas/coffee2400/slides/vswr-01.html
    >>and note that the VSWR of the common coffee can antenna is 2:1 (either
    >>25 or 100 ohms) near the band edges (2400/2450). Not exactly a
    >>perfect match but good enough as a 2:1 mismatch is "only" a 0.5dB
    >>loss. My guess(tm) is that either 50 or 75 ohm coax would work
    >>equally well with such an antenna.
    >>
    >>I have a fairly good supply of 75 ohm surplus CATV coax and have used
    >>it in a few installations where price was more important than
    >>performance. I've also designed and tested a few 75 ohm antennas. My
    >>50 ohm test equipment has input/output pads to deal with 75 ohm
    >>systems.
    >>
    >>A big problem is proper connectors. The common N, BNC, TNC, SMA
    >>connectors are available in both 50 and 75 ohm diameters. However,
    >>the more exotic R-TNC and R-SMA found on access points are not
    >>available. I have to use coax adapters (or butchery) at the access
    >>point and switch to standard connectors.
    >>
    >>As for the selection of coax cables, don't just grab a chunk of RG-6/u
    >>or RG-11/u and start using it at 2.4GHz. These have turned into a
    >>family of cables with radically different characteristics. Even the
    >>"satellite grade" of TV coax is only rated to 2GHz and is horribly
    >>lossy at 2.4GHz. For comparison, LMR-400 is 6.7dB/100ft, while the
    >>best RG-6/u is approx 11dB/100ft. For short runs, such loss is
    >>tolerable, but don't try it for long runs.
    >>
    >>Anyway, enough heresy. Use 50 ohm coax if possible, and only use 75
    >>ohms if you have a good reason to do so.
    >>
    >>
    >>--
    >>Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    >>150 Felker St #D http://www.LearnByDestroying.com
    >>Santa Cruz CA 95060 AE6KS 831-336-2558
    >
    >
    >
  8. Archived from groups: alt.internet.wireless (More info?)

    On Fri, 17 Dec 2004 08:16:07 -0500, "Dave VanHorn"
    <dvanhorn@dvanhorn.org> wrote:

    >Something to think about: Of the two, 50 ohm is optimized for power (at the
    >kilowatt level) and 75 ohm is optimized for low loss at high frequencies.

    It's the tradeoff between loss, power, and voltage. See:

    http://www.rwonline.com/reference-room/wired-4-sound/rwf_lampen_1_nov_5.shtml
    for the whole story. However, I should point out that if you build
    air dielectric coax from the copper water pipe sizes available in the
    1930's, you would get about 50 ohms.


    --
    # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
    # 831.336.2558 voice http://www.LearnByDestroying.com
    # jeffl@comix.santa-cruz.ca.us
    # jeffl@cruzio.com AE6KS
  9. Archived from groups: alt.internet.wireless (More info?)

    > http://www.rwonline.com/reference-room/wired-4-sound/rwf_lampen_1_nov_5.shtml
    > for the whole story. However, I should point out that if you build
    > air dielectric coax from the copper water pipe sizes available in the
    > 1930's, you would get about 50 ohms.

    Talk about hardline!! Of course you could water cool it :)
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