Balun for balanced lines measurements

[ea7qd]

Hi Bob,

In order to be able to carry out some measurements on balanced antennas, I have built a 1:1 balun. The balun is made up of a piece of PVC pipe 50 mm long and 16 mm diameter. In its inner part it has a ferrite bar of material 43. To check the balun operation I connected the balanced end to a non-inductive resistance of 50 ohm. The non balanced end I connected it to a BNC connector connected to the AIM 4170. The reference chosen was Zo 50 ohm. I tried between 1.8 and 30 Mhz and… just perfect! SWR 1:1 was in the whole band, no reactance and R 50 ohm. The balun was working properly so I was satisfied.

But I made another test.

How would it work with a 300 ohm charge?

I took away the 50 ohm charge and replaced it by a non-inductive 300 ohm charge. I set the analyzer at a reference value of Zo 300 ohm. What happened?

Results were depending on the frequency. SWR was not a straight line as before. Resistance values were different depending on the frequency, showing that impedance was complex. It was a real mess!

I think that with a 1:1 balun results should keep the same both for 50 ohm than for 300 ohm or any other non-inductive charge. Or am I wrong?



Greetings from Spain

[Bob]

Hello,

The cable used for making the balun will act like a transmission line transformer. When the load resistor is different from the Zo of this transmission line, it will not look the same at all frequencies, even if the resistor is non-reactive. Even a few centimeters of line makes a noticeable difference.

You can include the balun in the calibration process so its effect will be canceled out during the antenna measurement. Put the short, open and load resistor at the far end of the balun during calibration. This procedure will also help to compensate for baluns that are not ideal over the desired measurement range.

73/ Bob

[chuckshinn]

A couple of notes that I have been made aware of over the course of making balanced measures.

Firstly; displacement currents (capacitive coupling) between the clips or devices you use to attach to the DUT are capacitively coupled to the box the AIM is built into. To check your isolation between DUT and AIM, open your DUT end clip device and leave it hang open circuited. Then run your impedance scale up to 20K ohms. Sweep the frequency range you are interested in and observe how much capacitive reactance you get on the open lead. This will vary by frequency and lead length and composition of the choke. It will also change by how much rear apron choking you have applied to the wiring leaving your AIM unit. For instance I use Mix 31 snap its for upwards of 3 feet on both power and serial interface. At one point I had a 10 foot extension cord covered with bifilar wound snap it chokes on that cord and the serial cord as well.

Secondly; it is important to be aware of the nature of the coil form you are using. The use of PVC is extremely lossy at HF and can be the source of measurement errors. Personally I would eliminate the use of PVC in my test set up. There is a G station that went to the trouble of characterizing most common plastics used by hams for coil forms and PVC was horrible at HF. I cant recall his callsign but I found it on the web. I may be able to locate it on one of my PC's given some time. If I find it I will put it on my blog and post a link here.

If your DUT clips can maintain a solid 20K impedance while hanging your hot test clip open you have achieved a good and practical front panel choke. It isnt an easy task though.

[chuckshinn]

Sorry to reply to my own message but I found the reference I was talking about. A friend sent this to me about a year ago. I will include a quote to give you a sense of the quality of the material and then the link.
Quote________________________________

PVC...deserves special mention because it is a
thoroughly awful dielectric. The dielectric
losses which occur in plasticised PVC, in
particular, are greater than those which occur in
untreated wood. This is both unfortunate and
pernicious; because PVC is widely used as a cable
insulator in power distribution, audio, and
telephone applications. The ubiquity of PVC in
electrical service appears to lead amateur (and
some commercial) designers to believe that it is
suitable for RF purposes. The amateur literature
abounds with examples of coils wound on PVC
formers (drain-pipes etc.), sometimes with PVC
coated wire, all of which will exhibit abysmal Q
and are likely to melt in medium power (0.4 -
1.5KW) applications. For an illustration of the
effect of PVC formers on coil Q see reference
[14a and 14b]. Other failures known to the author
include: the use of PVC-covered tinned-copper
wire in balun transformers, the use of
PVC-covered twin-lead in antenna feeders, the use
of PVC sleeving for wire insulation in HF
linear-amplifier anode compartments, the use of
PVC electrical tape in high-field regions, and so
on (ad nauseam). The dielectric constant of PVC
is also highly temperature dependent, for which
reason PVC components should not be used in the
construction of oscillators and other
high-stability circuits.

End Quote_______________________________

The link is found here:
Although the link points to Part 6; IMHO the whole series is a treasure trove for RF Experimenters.
www.g3ynh.info/zdocs/comps/part_6.html

Best, Chas W7MAP/5