[R-390] R390 Baluns

[Bob Camp]

  Hi

The use of baluns with R390's is a topic that comes up fairly often.  
The first  implicit assumption is that it is necessary to match the  
input impedance of the reciever to the antenna in order to get  
optimum performance. The second implicit assumption is that the balun  
will handle the real impedances of the radio and antenna.

A lot of what we learn about antennas comes from working with  
transmitters. They don't really work the way we think but the math  
works out anyway.  In order to make a transmitter work properly you  
do need to supply it with the load impedance that it was designed to  
operate with. If we get to far away from that impedance bad things  
will happen. The issue is that the output stage in the transmitter  
has voltage and current ratings. If you exceed them smoke comes out.

Balun's are a bit like transmitters. You design them to work with  
specific resistive impedances on both side of the device. In a  
transmitter application loss in the balun equals heat in the core. If  
things get hot enough they explode. Balun explosions sound sound like  
a joke but they have killed some pretty famous people. The difference  
between 0.1 db and 0.3 db in a transmit balun can be a big deal.

So far all of this stuff is pretty well understood. It's also pretty  
important to understand so we spend a lot of time teaching it. A lot  
of people learn it as "gospel" without fully understanding that it  
applies specifically to transmitters.

Receivers are different than transmitters. One clue to this is when  
we rate them the units are microvolts and not something watts.  
Another more subtle clue is when we check the specifications on a  
well documented receiver (like the R390) the input impedance is given  
as a range rather than as a specific number. One way to look at this  
is that you get more voltage out of a given source if you run it into  
a high impedance load rather than a matched load. If volts are what  
counts then that's the way to get volts.

Receive antennas are different than transmit antennas. We rarely have  
the room to put up great big broadband antennas (like a rhombic). The  
antennas we work with are generally end feed or some kind of dipole.  
In either case we can't afford to cut one for each band we operate  
on. Since these antennas are commonly used there is a lot of data on  
what their impedance looks like over frequency. The data is available  
from a number of sources ranging from modeling programs to data on  
the web. The short version of the story is that these antennas are a  
lot more likely to be high impedance than low.

So far hopefully so good. This is all stuff That pretty much makes  
sense or can be quickly checked out.

Here's the first part that gets bizarre. When you match a source you  
cut the voltage available in half. That gets you 6 db of loss. When  
you do the match you cut the impedance in half. That ideally drops  
the noise figure by 3 db. The net result is that the signal to noise  
varies by about 3 db or so as we match or don't match the antenna. A  
0.1 db to 0.3 db variation in loss is not a big issue on a receiver.

One other way to look at this is that a 10 or 20 db signal to noise  
is needed for reasonable reception. With the detectors in an R390 the  
output signal to noise is approximately the input signal to noise.  
The difference between a 10 and a 10.2 db signal to noise is not  
going to be a big issue.

Transmit baluns have a tendency to saturate the core. When they do  
this you get some extra harmonics. A harmonic 40 or 60 db down is a  
problem. With a receiver a distortion product 120 or 140 db down can  
be a major issue.

So what's this all mean.

Receiving baluns are different than transmit baluns.

1) You would think that they could be pretty small. The distortion  
requirements make this less so than you would think. Big cores are a  
good idea.

2) The transmit formulas give you a required inductance. In a receive  
application the optimum inductance (and impedance) on a typical  
receive application is actually higher than for the transmit  
application (like two or three times higher). This is especially true  
since the inductance impact is greatest at the lowest frequency.  
That's also where the antenna impedance is highest.

3) With a R390 balance is an important part of the front end design.  
It also needs to be part of the balun design. This likely will drive  
you to a dual core design. The same impedance boost requirements  
apply to both cores.

4) Evaluating a transmit balun is fairly easy, run it and see if it  
gets hot. Since our ear responds to loud rather than good evaluating  
a receive balun with a real antenna is not so easy.

5) Multi wire magnet wire works pretty well for low impedance baluns  
and for transmit baluns. Twisted teflon insulated small diameter hook  
up wire works better for receive  applications.

That's the easy part.

Now you need a non-integer transform ....

     Take Care!

             Bob Camp
             KB8TQ