Question about SMA-SMA coaxial cable

I have a question about SMA-SMA coaxial cable offered by Ettus

Does this cable have an antenna effect?
In other words, if I use this cable, receive gain or some performance
measure is better than what I expected due to this cable becoming a kind
of
antenna.
Currently I’m looking for a certain cable which does not have such
effect
for very wide frequency range including very low frequency range i.e. DC
to
400-500 MHz

On Wed, 2011-06-08 at 17:50 +0900, Songsong G. wrote:

I have a question about SMA-SMA coaxial cable offered by Ettus

Does this cable have an antenna effect?
In other words, if I use this cable, receive gain or some performance
measure is better than what I expected due to this cable becoming a
kind of antenna.
Currently I’m looking for a certain cable which does not have such
effect for very wide frequency range including very low frequency
range i.e. DC to 400-500 MHz

No coaxial cable should act as an antenna except through shield leakage,
which is VERY low, especially at low frequencies. To verify that this is
the case, attach a 50 ohm SMA termination to the end of your coaxial
cable and measure the amount of signal coming into your USRP. It should
be next to none. If it isn’t, it’s possible your SMA cable is damaged
and has a disconnected shield.

–n

On 06/08/2011 12:03 PM, Nick F. wrote:

The exception to this is if the coax isn’t terminated in its
characteristic impedance. In which case, you get shield currents
flowing along with radiation from those currents. The magnitude of
those currents is first-order proportional to the
magnitude of the impedance mismatch.

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

On Wed, 2011-06-08 at 17:50 +0900, Songsong G. wrote:

I have a question about SMA-SMA coaxial cable offered by Ettus

Does this cable have an antenna effect?
In other words, if I use this cable, receive gain or some performance
measure is better than what I expected due to this cable becoming a
kind of antenna.

As was mentioned, the cable itself does not exhibit any significant sort
of
“antenna effect” unless it’s somehow damaged (or perhaps the grounding
is poor
on the USRP’s PCB). However, the cable absolutely can act as an
impedance
matching device if your actual antenna (or whatever is at the far end of
the
cable) isn’t 50ohms. Now, if the USRP’s impedance (looking back into
the
output) is precisely 50ohms, while the input impedance of the cable now
changes
with its length, the VSWR is – ignoring loss (it’ll be quite small in
short
cable) – the same and it doesn’t usually matter much. In actuality,
though,
the USRP’s output impedance isn’t exactly 50ohms across wide bandwidths,
and
hence you are actually matching better or worse to the load based on the
length
of the cable and can see a difference in signal strength.

While I haven’t actually experienced this myself with a USRP, I have
seen it on
other radios to the tune of, say, +/-3dB signal strength differences
depending
on the cable length used.

Currently I’m looking for a certain cable which does not have such
effect for very wide frequency range including very low frequency
range i.e. DC to 400-500 MHz

At very low frequencies, the approximation of coax cable’s impedance of
50ohms
is actually often not that horribly accurate. On the other hand, since
coax
lengths in terms of wavelengths are commensurately smaller as well, the
cable
itself tends to become more and more “transparent” and typically
non-50ohm
characteristic impedance doesn’t matter as much.

The standard “trick” for obtaining wideband input and output impedances
over
wide frequencies ranges is to use resistive pads (tee or pi) – a 6dB
pad will
get you at least a 12dBreturn loss, for instance; if you can afford the
extra
power to do this, it’s by far the easiest way to go. (You’ll note that
test
equipment like spectrum analyzers are almost always spec’d with an
internal 10dB
attenuator engaged!)

“Wideband,” “good low-loss matching,” and “easy” are a canonical “pick
any two”
meme of RF design. (Indeed, you actually hit some fundamental limits in
this
game – e.g., fundamental limits on the Q of electrically small antennas
are
depressingly low, and the Bode-Fano matching bandwidth limitations are
significant although typically not as troublesome.)

—Joel

2011/6/8 Songsong G. [email protected]


Seokseong Jeon (aka Songsong G.)


Discuss-gnuradio mailing list
[email protected]
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

Hello Songsong,

I have made an easy near field probe for such observations, it is easy
to
make!
You need a core (ferite or iron) on which sould be winded some
copperwire.
I have used a amidon FT37-61 core with 8 windings conectet to a BNC this
works
for frequencies up to 200MHz, but amidon has other cores suitable for
handling
greater frequencies. The next is you should have some thing for
meassuring
the
Voltage (use a scope or a simple detector).
So if you go along the coaxcable with the probe and you see an increased
voltage,
something would be faulty. :frowning:

regards
Josef V.

Just a note to those who spend $$$ on their antennas for trivial
tests.

The easiest antenna would be a 1/2 wave dipole, two leds soldered to
your coax (one to braid and the other to center)
Rip from your home electronics (DC-adapter etc) a ferrite and put it
close to your 1/2 dipole so currents will (may) flow on the inside of
your coax.
In my case I used a 1/4 wave copper tube (instead of a ferrite, sleeve
BalUn) when testing GSM @ 1.8 GHz
http://www.poes-weather.com/~patrik/usrp/1.8%20GHz%20Dipole-helix.jpg
Using no impedance match aint the end of the world since a 1/2 wave
dipole is ~70 ohm http://en.wikipedia.org/wiki/Dipole_antenna

To void other signals comming in you to your wave lenght, you could add
a 1/4 wave shorted stub

There are alot of documents on the web. Be some careful though and don’t
trust everything.

I bet, in most user cases an dedicated antenna (TX/RX) can be built
within an hour using < $5.
The good part here is, when you build your antenna yourself YOU learned
something (works or not).

A great software we use for dedicated antenna design is
http://www.eznec.com/

Good luck (luck is a bad word in our case)

Regards,
Patrik
----- Original Message -----
From: Joel K.
To: [email protected]
Sent: Thursday, June 09, 2011 6:33
Subject: Re: [Discuss-gnuradio] Question about SMA-SMA coaxial cable

On Wed, 2011-06-08 at 17:50 +0900, Songsong G. wrote:

I have a question about SMA-SMA coaxial cable offered by Ettus

Does this cable have an antenna effect?
In other words, if I use this cable, receive gain or some
performance
measure is better than what I expected due to this cable becoming a
kind of antenna.

As was mentioned, the cable itself does not exhibit any significant
sort of “antenna effect” unless it’s somehow damaged (or perhaps the
grounding is poor on the USRP’s PCB). However, the cable absolutely can
act as an impedance matching device if your actual antenna (or whatever
is at the far end of the cable) isn’t 50ohms. Now, if the USRP’s
impedance (looking back into the output) is precisely 50ohms, while the
input impedance of the cable now changes with its length, the VSWR is –
ignoring loss (it’ll be quite small in short cable) – the same and it
doesn’t usually matter much. In actuality, though, the USRP’s output
impedance isn’t exactly 50ohms across wide bandwidths, and hence you are
actually matching better or worse to the load based on the length of the
cable and can see a difference in signal strength.

While I haven’t actually experienced this myself with a USRP, I have
seen it on other radios to the tune of, say, +/-3dB signal strength
differences depending on the cable length used.

Currently I’m looking for a certain cable which does not have such
effect for very wide frequency range including very low frequency
range i.e. DC to 400-500 MHz

At very low frequencies, the approximation of coax cable’s impedance
of 50ohms is actually often not that horribly accurate. On the other
hand, since coax lengths in terms of wavelengths are commensurately
smaller as well, the cable itself tends to become more and more
“transparent” and typically non-50ohm characteristic impedance doesn’t
matter as much.

The standard “trick” for obtaining wideband input and output
impedances over wide frequencies ranges is to use resistive pads (tee or
pi) – a 6dB pad will get you at least a 12dBreturn loss, for instance;
if you can afford the extra power to do this, it’s by far the easiest
way to go. (You’ll note that test equipment like spectrum analyzers are
almost always spec’d with an internal 10dB attenuator engaged!)

“Wideband,” “good low-loss matching,” and “easy” are a canonical “pick
any two” meme of RF design. (Indeed, you actually hit some fundamental
limits in this game – e.g., fundamental limits on the Q of electrically
small antennas are depressingly low, and the Bode-Fano matching
bandwidth limitations are significant although typically not as
troublesome.)

—Joel

Forgot to say/suggest:

By adding a 1m (or so, length not that important) long coax to RX input
on FM (100 MHz band) improves alot (no dipoles, just straight).
Even if lick your finger and point it at the RX center you will receive
FM radio using USRPx.

There are lots of youtube videos that you might find interesting

COAX does ALWAYS act as an antenna (even how expensive it is).
In most cases it can be neglected

Patrik
----- Original Message -----
From: Patrik T.
To: [email protected]
Sent: Thursday, June 09, 2011 20:53
Subject: Re: [Discuss-gnuradio] Question about SMA-SMA coaxial cable

Just a note to those who spend $$$ on their antennas for trivial
tests.

The easiest antenna would be a 1/2 wave dipole, two leds soldered to
your coax (one to braid and the other to center)
Rip from your home electronics (DC-adapter etc) a ferrite and put it
close to your 1/2 dipole so currents will (may) flow on the inside of
your coax.
In my case I used a 1/4 wave copper tube (instead of a ferrite, sleeve
BalUn) when testing GSM @ 1.8 GHz
http://www.poes-weather.com/~patrik/usrp/1.8%20GHz%20Dipole-helix.jpg
Using no impedance match aint the end of the world since a 1/2 wave
dipole is ~70 ohm http://en.wikipedia.org/wiki/Dipole_antenna

To void other signals comming in you to your wave lenght, you could
add a 1/4 wave shorted stub

There are alot of documents on the web. Be some careful though and
don’t trust everything.

I bet, in most user cases an dedicated antenna (TX/RX) can be built
within an hour using < $5.
The good part here is, when you build your antenna yourself YOU
learned something (works or not).

A great software we use for dedicated antenna design is
http://www.eznec.com/

Good luck (luck is a bad word in our case)

Regards,
Patrik
----- Original Message -----
From: Joel K.
To: [email protected]
Sent: Thursday, June 09, 2011 6:33
Subject: Re: [Discuss-gnuradio] Question about SMA-SMA coaxial cable

On Wed, 2011-06-08 at 17:50 +0900, Songsong G. wrote:
> I have a question about SMA-SMA coaxial cable offered by Ettus
>
> Does this cable have an antenna effect?
> In other words, if I use this cable, receive gain or some 

performance
> measure is better than what I expected due to this cable becoming
a
> kind of antenna.

As was mentioned, the cable itself does not exhibit any significant 

sort of “antenna effect” unless it’s somehow damaged (or perhaps the
grounding is poor on the USRP’s PCB). However, the cable absolutely can
act as an impedance matching device if your actual antenna (or whatever
is at the far end of the cable) isn’t 50ohms. Now, if the USRP’s
impedance (looking back into the output) is precisely 50ohms, while the
input impedance of the cable now changes with its length, the VSWR is –
ignoring loss (it’ll be quite small in short cable) – the same and it
doesn’t usually matter much. In actuality, though, the USRP’s output
impedance isn’t exactly 50ohms across wide bandwidths, and hence you are
actually matching better or worse to the load based on the length of the
cable and can see a difference in signal strength.

While I haven't actually experienced this myself with a USRP, I have 

seen it on other radios to the tune of, say, +/-3dB signal strength
differences depending on the cable length used.

> Currently I'm looking for a certain cable which does not have such
> effect for very wide frequency range including very low frequency
> range i.e. DC to 400-500 MHz

At very low frequencies, the approximation of coax cable's impedance 

of 50ohms is actually often not that horribly accurate. On the other
hand, since coax lengths in terms of wavelengths are commensurately
smaller as well, the cable itself tends to become more and more
“transparent” and typically non-50ohm characteristic impedance doesn’t
matter as much.

The standard "trick" for obtaining wideband input and output 

impedances over wide frequencies ranges is to use resistive pads (tee or
pi) – a 6dB pad will get you at least a 12dBreturn loss, for instance;
if you can afford the extra power to do this, it’s by far the easiest
way to go. (You’ll note that test equipment like spectrum analyzers are
almost always spec’d with an internal 10dB attenuator engaged!)

"Wideband," "good low-loss matching," and "easy" are a canonical 

“pick any two” meme of RF design. (Indeed, you actually hit some
fundamental limits in this game – e.g., fundamental limits on the Q of
electrically small antennas are depressingly low, and the Bode-Fano
matching bandwidth limitations are significant although typically not as
troublesome.)

---Joel

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