TRI10 101520— The Golden Rules of Triband Yagi Installation
This articles is aimed primarily at novice and less experienced radio amateurs, with the goal of explaining in a simple and practical way some of the fundamental aspects of installing a triband Yagi antenna.
You don’t need to be an antenna theory expert: we will explain concepts such as height, wavelength, radiation pattern, elevation angle and SWR using clear and easy-to-understand language.
The examples refer to the Momobeam TRI10 10/15/20, but the concepts discussed can also help you better understand the behavior of other directional antennas.
How important is the height of a Yagi antenna?
Did you know that installation height can radically change the behavior of your Yagi?
When installing a triband antenna such as the TRI10 10/15/20, we should not think only about the antenna itself.
The antenna interacts continuously with:
– the ground
– the roof
– metal structures
– other antennas
– the mast and tower
– the coaxial cable
For this reason, the same antenna installed at 5, 10 or 20 meters can behave very differently.
And be careful: increasing the height does not simply mean “more dB”.
Height mainly changes the radiation pattern and elevation angle, two parameters that are fundamental for DX.
What is wavelength?
When discussing antenna height, it is not enough to say “the antenna is 10 meters high.”
We have to ask:
10 meters compared with which wavelength?
Wavelength changes with frequency:
Band Frequency Approx. λ
10 m 28 MHz 10.7 m
15 m 21 MHz 14.3 m
20 m 14 MHz 21.4 m
So, with a TRI10 installed 10 meters above ground:
– 10m (28MHz) → approximately 0.94 λ
– 15m (21MHz) → approximately 0.70 λ
– 20m (14MHz)→ approximately 0.47 λ
The same physical height therefore creates three completely different electrical conditions.
This is one reason why a triband antenna does not necessarily behave the same way on all three bands.
TRI10 at 10 meters: is it too low?
No.
A TRI10 installed at approximately 10 meters/33 ft can perform very well.
But we need to understand what happens.
At this height:
– 10m (28MHz) → approximately 0.94 λ
– 15m (21MHz) → approximately 0.70 λ
– 20m (14MHz)→ approximately 0.47 λ
The situation is therefore very good on 10 meters, while on 20 meters the interaction with the ground remains significant.
– 10 meters can be an excellent compromise when a higher installation is not possible.
But if DX is the main objective, we can do even better.
Tomorrow we will look at a particularly interesting height for the TRI10:
15 meters
Why is 15 meters such an interesting height?
Consider a TRI10 installed 15 meters above ground.
The electrical height becomes:
– 10 m (28MHz) → 1.40 λ
– 15 m (21MHz) → 1.05 λ
– 20 m (14MHz) → 0.70 λ
Notice something?
On all three bands, the antenna is in a very favorable condition.
For this reason, 15 meters is an excellent compromise for a TRI10 intended for both DX and general operation.
Of course, there is no “magic height”.
A well-positioned antenna at 15 meters, clear of obstacles, may perform better than an antenna at 20 meters surrounded by metal structures.
TRI10 at 20 meters: why is it so interesting for DX?
At 20 meters above ground:
– 10 m (28MHz) → 1.87 λ
– 15 m (21MHz) → 1.40 λ
– 20 m (14MHz) → 0.93 λ
We are therefore close to one wavelength on 20 meters.
This is a very interesting condition for achieving a vertical radiation pattern favorable to long-distance contacts.
It is also the height used by Momobeam for the published TRI10 gain figures.
But be careful:
20 meters does not automatically mean “X dB more gain”.
Height changes the radiation pattern and the interaction with the ground.
And that is exactly what we will look at in the next post.
Higher = more dB? NOT necessarily!
This is one of the most common questions.
“If I raise my Yagi by 10 meters, how many dB will I gain?”
The correct answer is:
– There is no linear meters-to-dB conversion.
When we raise the antenna, we change:
• the vertical radiation pattern
• the elevation angle
• the lobes
• the nulls
• the interaction with the ground
For DX, the amount of energy radiated at low elevation angles can increase significantly.
Therefore, the antenna can become more effective for a particular type of contact, without this necessarily meaning a constant increase in gain in every direction.
In RF, where the energy goes can often be more important than how many dB are shown on paper.
Is the ground an “RF mirror”?
In a sense, yes.
When a Yagi transmits, part of the energy travels directly toward the receiving station.
Another part is radiated toward the ground and reflected.
The receiving station can therefore receive:
direct wave + reflected wave
Depending on their phase, the two waves can:
– reinforce each other
– partially cancel each other
The result depends on:
antenna height;
frequency;
ground characteristics;
elevation angle.
This means that the ground is not simply something that sits “below” the antenna.
It is part of the electromagnetic behavior of the installation.
The real advantage of height: elevation angle
For DX, having good power is not enough.
It is essential to know at what vertical angle the energy is radiated.
A low-mounted Yagi can have a significant amount of its energy concentrated at relatively high elevation angles.
As the antenna is raised, the vertical radiation pattern changes and the main lobe can move toward lower elevation angles.
Why is this important?
Because lower angles are generally more favorable for long-distance contacts.
So:
HEIGHT → radiation pattern → elevation angle → DX performance
This, more than simply the number of dB, is one of the reasons why a high-mounted Yagi can be much more effective.
Why does SWR change when I rotate the Yagi?
This is a very interesting question.
A Yagi in free space should not significantly change its impedance simply because we rotate it.
But a real Yagi is never completely isolated from its environment.
If it is installed near:
– roofs
– metal structures
– other antennas
– cables
– solar panels
– towers
the situation changes.
When we rotate the antenna, its geometry relative to these objects changes.
Therefore:
coupling → phase → impedance → SWR
This is why a low-mounted antenna can show different SWR values depending on the direction in which it is pointed.
It does not automatically mean that the antenna is faulty.
Nearby metal can become a parasitic element
A Yagi can be perfectly designed and tuned.
But if it is installed close to a large metal structure, that structure can interact with the RF field.
The problem becomes particularly noticeable when the antenna is mounted low.
Think about:
– railings
– towers
– solar panels
– metal roofs
– other antennas
When we rotate the Yagi, the relative position of these objects changes.
Their coupling to the antenna can therefore change.
The result?
SWR can change with azimuth.
The best solution is not to “retune the antenna for every direction.”
When possible, the solution is to find a more open installation location.
Don’t forget the coaxial cable!
There is another element that can affect Yagi performance:
the coaxial cable.
Ideally, the coax should simply carry RF to the antenna.
But if common-mode current flows on the outside of the coax shield, the cable can become part of the radiating system.
This can affect:
impedance;
SWR;
radiation pattern;
behavior as the antenna rotates.
This is why the feed system is just as important as the antenna itself.
The TRI10 is supplied with a 3 kW RF Choke.
– A good choke helps prevent the feedline from unintentionally becoming part of the antenna.
How can you tell if your SWR is affected by the environment?
Here is a simple test.
Choose a frequency, for example:
14.200 MHz
Then measure SWR every 45°:
0° → 45° → 90° → 135° → 180° → 225° → 270° → 315°
Write down the results.
Then repeat the test on the other bands.
If you obtain something like:
Direction SWR
0° 1.20
45° 1.25
90° 1.45
135° 1.65
180° 1.30
225° 1.22
270° 1.35
315° 1.25
you have an indication that the environment is affecting the system.
Check:
– antenna height
– metal structures
– other antennas
– coax routing
– RF choke
Before modifying the antenna, analyze the installation.
The golden rule of the TRI10
A Yagi is not just the antenna. It is the antenna + the environment in which it is installed.
For the TRI10:
10 m → good height
15 m → excellent compromise
20 m → excellent for DX
But remember:
A TRI10 at 15 meters, clear of obstacles, can outperform a TRI10 at 20 meters surrounded by metal structures.
And if SWR changes as you rotate the antenna?
It does not automatically mean that the antenna is faulty.
It could be the environment, the ground, or the feed system.
Antenna design is only part of the result. Installation is equally important.
73 de Momobeam
