Cable impedance?

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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


Re: Cable impedance?
: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?


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


Re: Cable impedance?
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



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




Re: Cable impedance?
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
>
>
>
>
>>On 15 Dec 2004 08:13:33 GMT, roberson@ibd.nrc-cnrc.gc.ca (Walter
>>Roberson) 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
>
>
>


Re: Cable impedance?

>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

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


Re: Cable impedance?

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.





Re: Cable impedance?
On Fri, 17 Dec 2004 08:16:07 -0500, "Dave VanHorn"

>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


Re: Cable impedance?

> 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 :)




Re: Cable impedance?
On Thu, 16 Dec 2004 23:29:05 -0800, Jeff Liebermann

>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


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