# Antenna Analyze, or System Analyze?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CHUCKSTEIN, Oct 8, 2021.

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1. ### CHUCKSTEINQRZ Member

I am sure a write-up is here somewhere, but I cannot find it. Lets say I wanted to measure VSWR parameter of some arbitrary freq range. I can analyze just the antenna, or I can attach the antenna to a Tx with transmission line.

If I analyze just the antenna, will that VSWR look the same as connected to the Tx? Perhaps in other words, does Tx output stage affect overall "system" VSWR? I assume it would. So, if Tx is a variable in the VSWR equation, then why do we analyze just the antenna?

2. ### WA7ARKHam MemberQRZ Page

No. If the feedpoint impedance as measured right at the antenna is different than (50 +- j0) Ohms, the Transmission Line modifies the impedance that would be measured at the station end of the feedline.

A really good way to visualize this is to down load a free transmission line modeler created by a QRZ antenna forum regular @AC6LA. You can find it on his website at AC6LA.com

The program is called TLDetails.exe. Here is an example of how you would use it. Suppose you have an antenna that measures 86+j52 Ohms right at its feedpoint, and you are feeding it with 75ft of Rg-8X coax. You can ask TLDetails what your rig/tuner would see by entering the following data for say the 40m band:

This shows that if you measured the Z at the transmitter end of the coax, you would see ~30 - j24 Ohms, much different than what you would measure right at the antenna. Note that the swr at the transmitter is slightly reduced due to coax loss. Note that the coax delivers only 82W to the antenna (starting from 100W), and so on....

Last edited: Oct 8, 2021
3. ### CHUCKSTEINQRZ Member

So if there's a Tx+antenna w/o feedline, VSWR is always 1:1 if the inked spec of each is 50-Z. What if Antenna is 50-Z but is highly inductive, or highly capacitive? If the Z's match then the only diff will be apparent power?

4. ### WA7ARKHam MemberQRZ Page

Look at post #2 again, and then ask your question in the context of what you want me to put into TLDetails, or better yet, get it and do it yourself...

5. ### KE0NSKHam MemberQRZ Page

When tuning an antenna system with coax, the VSWR dip is the only true indication of resonance when using a VNA.

6. ### W9IQHam MemberQRZ Page

An SWR dip does not indicate resonance regardless of the instrument used to measure SWR. An SWR dip simply indicates a complex impedance that is closer to the feedline impedance than the "non dip" measurements. In fact, it is possible to get an SWR dip that has more reactance than a "non dip" region.

Resonance means no reactance (or more practically very little reactance). It does not mean the real part of the impedance is 50 ohms but rather that the complex term (X or j) is (near) zero.

- Glenn W9IQ

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7. ### WB5YUZHam MemberQRZ Page

As you may have understood by now, if the antenna is not very close to 50 ohms, then a 50 ohm feedline will act as a transformer and obscure the actual impedance of the load.

This is why we often measure VSWR at the shack end of the feedline, to analyze the overall system impedance, not just the antenna impedance.

Now, if you want to know the antenna's impedance, you can cut a bit of feedline that is either an electrical half wave (don't forget the velocity factor!) or a multiple of a half-wave. Although this solution is theoretically only accurate on one frequency (that on which the feedline is an exact multiple of a half-wave), in practice you can measure a few kHz up and down and still get a useful reading.

The half-wave of feedline trick is how most of us used to do it in the days before computer modeling and apps. Now, with nano VNAs, there are other, more convenient ways to do it, but I don't know what they are.

8. ### W9IQHam MemberQRZ Page

Due to losses in the coax or other transmission line, generally the SWR at the transmitter/source end will be less than the antenna/load end. When the SWR is low or the matched coax loss is low, the effect will be minimal. But as coax loss or SWR is increased, the effect becomes more pronounced.

One interesting effect is that for a given matched transmission line loss, there is a maximum SWR cannot be exceeded at the transmitter end regardless of the actual SWR at the antenna end.

- Glenn W9IQ

9. ### KE0NSKHam MemberQRZ Page

Call it what you'd like, but the VSWR dip indicates where the antenna radiates most efficiently. If you measure something different, you are misinterpreting your data.