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Author Topic: estimate overall efficiency of transmatch/coax/ant  (Read 1733 times)

Posts: 2

« on: April 17, 2009, 06:28:01 AM »


I  am trying to understand an easy way to estimate the overall efficiency of my transmatch/coax/antenna system.    Please read the following and tell me if I am all wet,  and/or what I am missing..


I use a homebrew Vertical Coaxial Dipole.    The overall length of the dipole is 6.94 meters.   This is strictly a single band antenna.
(I realize 15m band is not good right now, but I am using this method to evaluate on other bands)

The dipole is made of two pieces of aluminum  tubing each approx 11 feet (3.4 meters) long,  and a 4 foot (1.2 meter) long piece of PVC pipe.  The 2 aluminum  tubes are inserted into the PVC pipe,   leaving about 3 inches (75 mm) so the tubes do not touch each other.

The feed line is RG-8 coax.  The feedline goes up through the center of the lower length of aluminum tubing to the feed point at the center of the dipole,  inside the PVC pipe.   The center conductor of the coax is connected to the upper length of aluminum tubing (there is no balun device at the feed point).  The outer braid from the coax is connected to the lower aluminum tubing.  The lower half tubing is not electrically connected to anything other than the coax outer braid.
 i.e. no ground connection.

The dipole is mounted in the vertical position,   with the lower end of the bottom aluminum tubing 1 meter off the ground.

The feedline is 50 feet of RG-8 coax to a  transmatch w/ SWR meter.

The transmatch is attached to the transmitter with about 1 foot of RG-58 coax.

The  transmatch is adjusted to give SWR 1:1   (at the transmitter)


I estimate the power radiated by the antenna as follows:

1.   Model the antenna with MMANA-G.  
        using the 'real ground' option
        MMANA-G reports calculated SWR = 1.74 at  21.050 MHz

2.   Determine coax loss using “Coax Calculator” at
        Input to Coax Calculator
        Line Type:      Belden 8237  (RG-8)
        Line Length:   50 feet
        Frequency:   21.050 MHz
        Load SWR:   1.74
        Power In:   100 Watts

        Matched Loss:   0.41  dB
        SWR Loss:   0.058  dB
        Total Loss:   0.468  dB
        Power Out:   89.782  Watts


3.   With 100 watts transmitter output, the power radiated by the antenna is 89.782 watts.
        Which means that the overall efficiency of the system is 89.782 %

My question is:

Is my approach to estimate power radiated by antenna realistic?

Is it reasonable to use the 1.74 SWR from MMANA-GAL,  as the “Load SWR” in the “Coax Calculator”?

does the bottom end of dipole at 1 meter above ground introduce ground losses that my method does not include?  or does it simply affect the radiation pattern from the dipole?

What am I missing?   Please be gentle,  but not kind.

Sam Winstead

Posts: 625

« Reply #1 on: April 17, 2009, 07:48:40 AM »

Since you are modeling a vertical antenna system low to the ground, the ground chacteristics are very important.  Unless you have some way of actually measuring the ground conductance and dielectric properties, you are just guessing.

In the end, what really matters is how much RF is radiated at a particular elevation angle and azmuth for which you intend to communicate.

You will learn something useful about your system if you model two or more antennas that you could reasonably put up relative to each other and pick the best one for the job.

Modeling is very seductive, but the quality of the ground modeled is always a big guess.  So in my opinion the best  approach is to model on a relative basis and especially to pay attention to the radiation pattern produced.  You can learn a lot about useful approaches to modeling by reading the material at the W4RNL (Cebik) site.

Lastly, if you figure an S unit on a typical RX might be a step of 3 to 6 dbV, you might vary the power by 1/2 and see one S unit difference.  If you had two antennas that varied by that amount and you switched between them durning an HF QSO, you would be hard pressed to tell the difference, given normal QSB.

Posts: 14491

« Reply #2 on: April 17, 2009, 11:25:23 AM »

Your efficiency figure assumes that the only loss in the system is the loss in the coax. I think you'll find that there are other losses involved, especially with it mounted near the ground.

I would question how accuratly the modeling programs's SWR actually is, again especially with it being close to the ground. Why don't you put a meter in line close to the antenna and actually measure it?

I agree that a couple dB one way or the other on HF is pretty hard to detect from a practical standpoint. The exercise is a good learning experience though.

Bob  AA4PB
Garrisonville, VA

Posts: 2

« Reply #3 on: April 17, 2009, 06:25:45 PM »

I used Ground Conductivity = 15 in MMANA-G

I got The value of 15 from a map of conductivity at
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