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How can I measure antenna input power?

 
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garyspang
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Joined: 11 Mar 2018
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Location: Clemson area, South Carolina

PostPosted: Sun Mar 25, 2018 3:29 pm    Post subject: How can I measure antenna input power? Reply with quote

I would like to measure the power entering/leaving my antenna, at the antenna.

Available is a 100Mhz Techtronix digital oscilloscope, an RF current transformer mounted along with a resistive voltage divider in a small metal box.
The present system has a VSWR usually below 1.1 from 160 to 6 meters between the transceiver and the auto-tuner at the antenna on all the Amateur Radio HF bands. How this was achieved and verified experimentally is available upon request in the form of a two page article.
The RF power flow is from the ICOM-7100 transceiver at the driver position of our motor home, to 40 feet of 50 ohm coaxial transmission line, to a high resolution auto-tuner MFJ-939, then six inches of coax to an Auto-tuner Extender MFJ-914 with a 1:4 impedance setting, that feeds a 31foot Eagle One vertical antenna mounted on the ladder on the back of the motor home. Several ground straps go from the tuner extender and the auto tuner to the motor home's 30 ft steel chassis. Without the auto-tuner, 160 and 80 meters has such a severe VSWR, that the transceiver shuts down using its protective circuits.

The quandary:
In light of the initial sky high high VSWR at 160 and 80 meters, that has been totally eliminated as seen by the transceiver and coax feed line, with almost 99% of the real RF energy going into the auto-tuner (neglecting its losses), what happens to the power? Will it really now leave the antenna at 160 and 80m?
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R. Fry
Antenna Theory Regular


Joined: 06 Jun 2011
Posts: 46
Location: Illinois USA

PostPosted: Tue May 22, 2018 11:36 am    Post subject: Re: How can I measure antenna input power? Reply with quote

garyspang wrote:
I would like to measure the power entering/leaving my antenna, at the antenna. ...

Power entering antenna system = IČR

where
I = r-f current across antenna system feedpoint terminals
R = the real term of the impedance at the antenna system feedpoint

Power radiated by antenna system = Input Power x (Rr / Rt)

where
Rr = radiation resistance of antenna system
Rt = sum of radiation resistance and the other real resistances of the antenna system, such as the connection to r-f ground (typically Earth), and the ESRs of any loading coils included in the antenna system
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garyspang
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Joined: 11 Mar 2018
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Location: Clemson area, South Carolina

PostPosted: Tue May 22, 2018 2:00 pm    Post subject: power at the antenna, will this approach work? Reply with quote

I do not have a way to obtain some of the parameters mentioned in the reply.

I will calibrate my toroidal current and resistive voltage sensor with an accurate Motorola dummy load at an 80m frequency. Then I will measure the amplitude and phase angle between the voltage and current at that frequency at the antenna terminals. Using conventional calculations, the real and reactive power will be calculated. The real power will be the power that could leave the antenna, and the reactive part is just circulated between the antenna and the tuner.

I hope this would lump all the mentioned parameters and represent the desired real power input to the antenna. How much is actually radiated may be another issue.
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R. Fry
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Joined: 06 Jun 2011
Posts: 46
Location: Illinois USA

PostPosted: Wed May 23, 2018 10:18 am    Post subject: Re: power at the antenna, will this approach work? Reply with quote

garyspang wrote:
I do not have a way to obtain some of the parameters mentioned in the reply. ... I hope this would lump all the mentioned parameters and represent the desired real power input to the antenna. How much is actually radiated may be another issue.

Method of Moments software such as NEC4+ (Numerical Electromagnetics Code) can supply the parameters needed to calculate the radiation efficiency of an antenna system on a given frequency, including:

» the r-f resistance of a network of radial wires buried in Earth of a specified conductivity and permittivity
» the total radiation resistance of the above-ground, radiating conductor(s)

NEC4.2 also will calculate/plot the gain of the radiation pattern envelope, and the field intensity it will produce at a given physical location, for a given applied input power.

Below is a graphic showing two examples of this, for the antenna system and conditions stated there:



Another NEC4.2 analysis showing the fields radiated by the system stated there:

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garyspang
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Location: Clemson area, South Carolina

PostPosted: Wed May 23, 2018 7:07 pm    Post subject: final radiated power approximation Reply with quote

Thanks R. Fry
Great analysis and suggestion. I will use it as a guide.
I am not sure that the model illustrated by your suggestion is applicable: The antenna is a 32ft vertical inside a telescoping fiberglass tube, but it defies al other normal construction of the stated model, by having no ground radials, only a 30 foot motorhome chassis and body framework four feet from the antenna base and about two feet above ground.
Therefore an experimental method preferably independent of frequency would be preferred. Measurements could be made at all amateur bands and relative radiation power/efficiency/field strength between bands determined. That would totally satisfy my curiosity.

Please lets remember that it started with the tuner at the base of the antenna making it look to the transmitter as a near perfect antenna (VSWR < 1.2 at 160, 80, 40, 20, 17,10m). But we know this can't be, therefore this sequence of desired measurements is to satisfy my curiosity as to what is really happening along the way, experimentally verified.
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PetApp
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PostPosted: Sun May 27, 2018 3:51 pm    Post subject: Reply with quote

Use a power meter or a spectrum analyzer to measure the power at the antenna connector.
For accurate radiated power measurement you need an anechoic chamber or an open-site setup.
Use a reference antenna, and a calibrated dipole antenna for substitution with your antenna. In this way you get your antenna gain in dBd (referenced to a dipole).
Knowing the power into the antenna and the antenna gain you can calculate the radiated power.
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garyspang
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Location: Clemson area, South Carolina

PostPosted: Sun May 27, 2018 11:36 pm    Post subject: Reply with quote

Hi PetApp
If the frequencies had the wavelength of 2 meters or less, your approach is achievable. Unfortunately the amateur radio frequencies of interest are in the HF spectrum ranging from 160 meters to 10 meters in wavelength. A dipole at the longer wavelength would be a real bear to fabricate and to use as a tool.

Also measuring the power input at the antenna terminals is exactly what I whish to do (after ARRL Field Day). The only way I can imagine the tuner making the antenna impedance look non-reactive is by automatically adjusting the internal components to achieve the antenna's complex conjugate along with a conventional transformer step up/down function to arrive at 50 ohms for the match to the coax. The actual impedance at the antenna can be all over the place and would be unsuitable for normal power measurement devices since the ones I know, rely on a standardized line impedance (Z=50 ohms for Hams) which is not at the tuner output, nor at the antenna input.
Short of VSWR interference, the only way I can understand to achieve the measurement is by the use of separate antenna input terminal vector current and voltage measurement.
Unfortunately no one has yet responded to the validity of my simplistic approach. So I will wait for either a viable or a ridiculous measurement result (after ARRL Field Day because everything has been weather proofed).
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