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Telamon

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wrote:

On Dec 21, 6:26 am, wrote:
Dear All,

I have some beginners questions regarding antennas:

*** Why is it difficult to design and construct HF transmit antennas
for high power PEP, compared with low power 100 W?
Conceptually what is the difference between the two in design,
material construction?
Why do people find the high power harder to design and construct?

*** I understand that omnidirectional antennas transmit in all
directions with almost equal gain. Is it hard to design HF antenna
with directional gain in one direction and minimum in other
directions?
How is this done( conceptually): electrical length, material
contruction?

*** HF wire antennas: can these be for high power transmission and
directional gain ( guess the would always be omnidirectional)?

*** How is angle of transmission with the horizon (elevation)
controlled in a HF antenna ?

*** Can receive antennas also be diectional or are they always
omnidirectional receiving from all sides?
Do receive antennas have power ratings?

Thank you

Vijay

It is very simple.

Voltage.

The higher the power the higher the voltage.
100W into 75ohms gives 86V.
200W @ 75 Ohms = 122V
500W @ 75 Ohms = 196V
1000W @ 75 Ohms = 273V

The current also goes up with power.

Higher voltage mean heavier duty construction.

The voltage show are at the terminals of a simple dipole.
The voltages at the ends of a dipole will be much higher.
Receiver antennas don't have power ratings.

Although the local 146.76MHz repeater has 5W from a
collocated FM BCB at the diplexer input.

The "National Electric Code" has some requirements
for receive antenna conductor size that no one bothers
with. I don't have my set of NEC rules handy, but the
wire size was picked for mechanical strength rather then
power handling capacity.

This is all correct. The ramifications a

1. Even with increased wire sizes there will most likely be increased
losses due to skin effect. You lose effective transmit power.

2. Antenna and transmission line with be dissipating more power due to R
losses and heating up. You must design to dissipate this heat or things
will melt.

3. Because you have increasing voltages you have to pay increasing
attention to terminal spacing and dielectric breakdown in any part of
the transmission line or antenna. You might have to do things like round
the ends of antenna elements.

4. VSWR becomes more of an issue with higher power. Smaller increases in
VSWR can cause dielectric breakdown in some part of the antenna system
that under normal circumstances would not be an issue. In other words
the antenna system becomes more sensitive to changes that even weather
can bring about.

--
Telamon
Ventura, California

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