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Is it possible to measure the actual power getting to the antenna with H4 model? I can't seem to find any documentation on this. Power is 4 watts.
Thanks
Steve KE8VHY

you can measure your antenna ... (swr) and you can measure cable loss (s21)

then you can do some math to see what your antenna gets from your trx (and what is your receiver getting on the rx side)

dg9bfc sigi

Am 11.03.2023 um 22:56 schrieb Steve Bowers:

Much obliged

Thanks
Steve

The nanovna is a great tool to measure this; really what it can measure is
the loss in your feed cable and the fraction of reflected power from the
antenna feedpoint.
An easy way to measure the cable loss is to disconnect the cable at the
antenna feedpoint, and from the shack end connect the nano and use the
Measure -> Cable function. You read the cable loss directly on the screen
(no need to do an S12 measurement). The frequency range needs to be set so
the smith-chart trace goes at least one-half circle; if it spirals in many
circles, it is best to reduce the upper frequency so there are only a
couple of circles to improve accuracy.
And as Steve noted, do an SWR measurement at the desired frequency (or to
directly read dB, use the Log Mag return-loss display).
With those measurements you can easily calculate the loss in power getting
to your feedpoint, and the amount being reflected back, to give an estimate
of the effective power at the antenna feedpoint.

Steve,

Yes you can do the calculations to find the through power to the antenna with the aid of a NanoVNA. Some things to consider.

- Some transmitters will automatically slowly reduce their Tx power as the VSWR goes beyond a certain threshold. My Icom starts cutting back when the VSWR at the radio terminals goes above about 1.5

- Power will be lost in the transmission line to the antenna and this will increase with distance and VSWR. You can get a good estimate of the loss due to the transmission by several methods. Here are the ones I use. Calibrate you VNA and do an S21 measurement between CH0 (Port1) and CH1 (Port2). Find the loss at the frequency of interest using the marker and jog switch. If the coax feedline is already installed disconnect if from the antenna and measure the Return Loss at the frequency of interest. Record it as RLopen. Then short the far end and measure again. Record this as RL short. A good estimate is feedline attenuation is equal to (RLopen+RLshort)/4 for that frequency. The third method if you have a nanoVNA with a cable loss tool is to use that menu item.

- You need to know the VSWR at the antenna in order to calculate the through power. You can measure VSWR directly at the antenna or use one of several other methods to calculate it. One method is to "de-embed the cable" by doing your SOL calibration at the end of the cable. This requires an assistant or a olt of running back and forth to do. Once you have de-embedded the cable the VSWR measurements made in the shack are very close to what is happening at the antenna feedpoint. The other method is to use one of the nomographs available from the ARRL and others that do the calculation based on attenuation of the cable. I have attached one for you reference.

- When the VSWR at the antenna feedpoint is known the through power can be calculated or read from a table like the one attached. It gives what percentage of the power arriving at the feedpoint goes through and how much is reflected back.

- Here is an example calculation for you. It does not account for connector loss and transmitter foldback and assumes that the transmitter tuner source impedance is 50 ohms. If 4 watts is transmitted on a cable that has 3 dB of attenuation at the frequency of operation only 2 watts arrives at the feedpoint. If the VSWR is a perfect 1:1 then 2 watts goes into the antenna. If the VSWR is 3:1 (Return Loss is 6 dB) then 75% or 1.5 watts of the arriving power goes into the antenna and 25% or 0.5 watts goes back down the cable to the transmitter. As it goes back down the cable it is attenuated by 3 dB so .25 watts is absorbed by the transmitter or tuner source impedance. The total power dissipated in the transmission line as heat is 2 +.25 - 2.25 watts.

Roger