Dummy load and return loss bridge?

[elroy]

I've been seeing return loss bridges of this type:

http://transverters-store.com/rf_bridge/rf_bridge.html

on eBay and elsewhere, and have been trying to get more educated on how they are used to characterize antennas.

In the writeups and videos I've seen, a signal/noise source is connected to the input, an instrument like a spectrum analyzer is connected to the output, and the antenna being evaluated is connected to the DUT port.

I noticed, however, that some of the bridges being sold (such as the one linked above) have an additional port marked REF. I asked one eBay seller about it, and he said it was for a dummy load; for additional information, he said "google it."

So I'm just a little unclear on how/why you would use a dummy load while testing an antenna connected to the DUT port. Can anyone help me out here, or point me to anything that covers this specific point?

[hendorog]

The way these bridges work is by measuring the in-balance of the bridge - which in real terms is how close the device attached to the test port is to 50 ohms. The bridge is balanced when the both the DUT and the reference have the same impedance - nominally 50 ohms.

So to do the comparison you need something which is known to be 50 ohms to act as a reference.
Some bridges are built with a 50 ohm resistor built in as the reference. Others just have a port where you attach your own reference - in this case you use a 50 ohm dummy load to provide the 50 ohm reference.

[elroy]

Thanks, very clear!

[IconicPCB]

You could make it yourself with a bit of care and attention to detail

Yo could expect better than 30dB directivity that is to say You could measure return loss up to 30dB and more than 500MHz with the unit depending on the detector You use.

[Tony_G]

While not the same design, this article has some background info on the broad idea and usage of a SWR bridge:

http://www.w1ghz.org/QEX/A_UHF+_VSWR_Bridge.pdf

TonyG

[elroy]

While not the same design, this article has some background info on the broad idea and usage of a SWR bridge:

http://www.w1ghz.org/QEX/A_UHF+_VSWR_Bridge.pdf

Thanks!