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 -----
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.
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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