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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.
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).
Good *luck* (luck is a bad word in our case)
Regards,
Patrik
----- Original Message -----
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 Gee 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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