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How to measure the input impedance of a receiver ?

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Externet

Well-Known Member
Hi.
For a receiver that tunes from 174 to 800 MHz; What equipment is needed to measure its (antenna) input impedance and how should be done ?

I do have a 1GHz spectrum analyzer and signal generator, but afraid of blowing the front end of the receiver, and the proper procedure of measurement/plotting the whole span in 10 MHz steps. Suggestions please ?
 
If you want to resolve the impedance into its resistive and reactive components, the obvious thing to use would be a VNA (Vector Network Analyser), but you do not have one of those available.

Maybe the next best thing would be to use an RLB (Return Loss Bridge) which will give you the return loss (in dB) from which you can calculate the VSWR relative to some standard value, usually 50 Ohm.

A return loss bridge is very easy and cheap to build, but will need some care in construction if you are going up to 800MHz.

As a point of interest, why do you want to measure in input impedance ?

JimB
 
Thanks.
I do not have a network analyzer nor know who may have one :( I have a MFJ-249 that reads impedance but works under 174MHz and I do not think is intended for other than antennas. My Motorola R2600 analyzer fits the whole frequency span but does not feature impedance readings.
Would such network analyzer instrument do a sweep on a chosen span of frequencies and display a frequency versus impedance chart ; or would have to be manually stepped and noting the readings ?

Am after learning the true input impedance charts of different ATSC television tuners. Am convinced they are much higher than 75 ohms, and have been unable to find it in their published data, in order to design some antennas.

Would anyone have a network analyzer instrument and run a sweep on any television RF input ?
 
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My guess, is you need to start with a dual directional coupler such as: **broken link removed**

and two RF power meters. Then as Jim said use Return loss to back out Z.

My guess is that the antennas have a higher Z than 75 ohms. Probably more like 100,

What i don;t know is is you have to use say an 8VSB power meter if in the US. Probably not.

EDIT: Supposedly, this, (A return loss bridge) is better: https://www.google.com/url?sa=t&rct...gb89fQ2WCGgcFeA&bvm=bv.73231344,d.cWc&cad=rja
 
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Not exactly an answer to your post, but perhaps some useful FCC test info (.doc file) from this site:
http://www.google.com/url?sa=t&rct=...hcuwJWSXwnLwagBHwiOmsTw&bvm=bv.73231344,d.aWw

An excerpt:
ATSC Impedance.JPG



Also this (home brew antennae rigs, also with useful info), from this site: **broken link removed**
 
Maybe the next best thing would be to use an RLB (Return Loss Bridge) which will give you the return loss (in dB) from which you can calculate the VSWR relative to some standard value, usually 50 Ohm.

A return loss bridge is very easy and cheap to build, but will need some care in construction if you are going up to 800MHz.
Ebay has a Return Loss Bridge for $120us. (500mhz) and some better ones for $200 and $300. If you have $800 there is a 6ghz version.
 
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