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-   -   We know SWR, but what is Zx Rs and jX? (https://forums.radioreference.com/amateur-radio-antennas/345005-we-know-swr-but-what-zx-rs-jx.html)

NC1 12-24-2016 8:40 AM

We know SWR, but what is Zx Rs and jX?
 
I am having a discussion about the Surecom SA-250 Antenna Analyzer with a friend, and he seems to have a difference of opinion as to what those last 3 abbreviations mean.

Being that my theory is a little rusty, I am beginning to question if I am correct or getting it confused with something else. A good hour in a google search has ended in futility because of all the results either want me to buy something, or gives me a multitude of other non-related results.

If somebody could clear this up, it would be greatly appreciated.

N4GIX 12-24-2016 11:02 AM

Quote:

Originally Posted by NC1 (Post 2687016)
I am having a discussion about the Surecom SA-250 Antenna Analyzer with a friend, and he seems to have a difference of opinion as to what those last 3 abbreviations mean.

Zx is the characteristic impedance, i.e., the square root of L/C (inductance/capacitance).

jX is inductive reactance (if +) or capacitive reactance (if -)

Rs is the total resistance of the antenna system

In any case, I highly recommend reading this ARRL book:

Understanding Your Antenna Analyzer
https://www.scribd.com/doc/314785439...tenna-Analyzer

NC1 12-24-2016 12:40 PM

Quote:

Originally Posted by N4GIX (Post 2687115)
Zx is the characteristic impedance, i.e., the square root of L/C (inductance/capacitance).

jX is inductive reactance (if +) or capacitive reactance (if -)

Rs is the total resistance of the antenna system

In any case, I highly recommend reading this ARRL book:

Understanding Your Antenna Analyzer
https://www.scribd.com/doc/314785439...tenna-Analyzer

Thank you for clearing that up. I was a little rusty on the Zx, but I was correct on the other two.

That link looks like a very worth while read so I can brush up a bit on what I have forgotten over the years. When you don't use it very often, it tends to get in the back of the line and all but forgotten

I appreciate your Christmas gift, hope you have a good one and get a shiny new rig under the tree tomorrow morning.

Mikejo 03-16-2018 4:17 PM

I Bought a Surecom SA-250 recently, and I'm trying to better understand the other parameters the Analyzer shows me.

I have a Dipole Antenna I recently made cut for VHF High 160Mhz range for listening to Railroad Frequencies on my scanner.

When I test it on my Analyzer I get VSWR 1.12, Zx 47.0, Rs 63.9, and jX -43

What are the other parameters telling me about this particular antenna?

For instance is a Zx of 47.0 a good thing or a bad thing? If bad, what would be good?

How about Rs of 63.9 ? Good/Bad? If bad, what would be good?

Finally, jX of -43 Good/Bad? If bad, what would be good?

(I'm not a HAM, and I don't have an ARRL Handbook, and if I did I would probably not understand it anyway).

I just would like a laymans term explanation ( if possible), to know if any of these other parameters could help me make adjustments. or indicate a cable problem and the such.

n0nhp 03-16-2018 7:06 PM

Much of that is going to depend on the receiver you are using.
If you are using a scanner or other wide band receiver, your input impedance to the radio is not going to be a "perfect" 50 ohms. Even with a dedicated VHF High/ high split radio the receive impedance is pretty problematic.
For a transmitter however you really want the transmitter output impedance to match the input impedance of the antenna so you are wasting as little power as heat as possible. Even that said most of today's wide-band transmitters are very tolerant of a very wide impedance mis-match.

Zx of 47.0 = 50 ohms within any tolerance. Your antenna is well matched as is your coax.
Rs of 63.9 is again showing close enough to 50 Ohms to be within tolerances
jX of -43 is showing that the the antenna is showing capacitance may be a bit high. Could be a sign you are using slightly under-sized coax and could possibly be losing some signal there or it may just mean that the over-all design is capacitive. You could take the analyzer to the roof and see if things change.


One of the joys of building your own antenna systems is seeing where the theory and practice converge and diverge.

On receive the things that will make the most difference is altitude and transmission line.
A paper clip stuck in the connector with a good line of sight will out perform the highest dollar antenna that is stuck down in the attic.

Good luck
Bruce

Mikejo 03-16-2018 7:36 PM

Thanks Bruce, (I am well aware that I really don't need to have everything perfect with a receive Only antenna). I'm glad you gave me the information I was looking for, for both impedance and resistance I would just like to know where one should draw the line as to where it would be considered bad; let's say for the resistance to be "greater than "70"? Would that be where the line should be drawn?

Regarding "JX", let's say it's showing The antenna to be more capacitive than inductive, what could I do to change that? Is there a way to add either inductance or capacitance to straighten it out? Would this mean just using an antenna tuner to accomplish such a thing or could I add electrical components in the line to accomplish the same thing?

I know it's only a scanner antenna, but what I'm really interested in doing is making it as sensitive as possible, because the area I live in makes it difficult to hear anything but the dispatcher. I would like to try and see if I could pull in some of the actual trains and hear them as well.

n0nhp 03-16-2018 8:07 PM

The capacitive reading could (one reason I suggested testing at the antenna) be due to coax that is showing a fairly high capacitance at that frequency. This is why the higher the frequency and the longer the coax runs, the larger (for the most part) the coax used is. Using the appropriate formula (and no I don't remember it (Electronic tech classes were 40+ years ago) you can calculate the attenuation of the signal at the frequency you are interested in. Easier is to look at the spec sheets and see what the manufacturer specs are.
Adding inductance will probably not help as you would be putting a coat of paint on to make the meter tell you what you want but not actually curing the problem (If any).

The dispatcher is using a remote base placed on a tall tower or mountain top where at least in Colorado the tracks are almost always within spitting distance of a stream or river. Not conducive to distant comms.

Bad would be where whatever skyhook you had in the air didn't pick up anything.

Will spending some money on better coax or a higher gain antenna help?
I can't answer.
I have spent a lot of money on aluminum and copper pieces that are now playing host to insects in the storage sheds when they didn't work as well as what I had up. But I always have hope that the next experiment will be the best thing since sliced bread! :-)

Good luck
Bruce

Mikejo 03-16-2018 8:57 PM

Well, the the analyzer I have can't test the coax( at least I don't think I can) , but I can test the antenna at the other end. I'll see if it's a tad too short for the freq I want, if so, I can quickly cut a few more elements and start over.

Thanks for the help!


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