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

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?

Who says they find it harder to design for high power? The *design* is
exactly the same for high/low power. Only the *implementation* is
different. With high power, you will have greater current or voltage
which will require a larger conductor or greater insulation. Example: A
dipole fed with ladder line. The typical ladder line sold for ham use
will break down (arc over) at around 1200 volts (IIRC). On some
frequencies, the impedance of the antenna will be quite high. By Ohm's
Law, E(voltage) = I(current) X Z(impedance). If the antenna has, say
Z=2000 ohms and due to running high power, you have an amp of RF current
flowing, you will have 2000 volts (1 X 2000) which will exceed the
breakdown rating of the ladder line. Please read the ARRL 'Antenna Book'.

*** 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?

Very briefly, several ways: multiple phased vertical antennas or a Yagi
antenna will give you directionality. Please read the ARRL 'Antenna Book'.

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

Yes, there are directional wire antennas, for example the Sterba Curtain
or Rhombic. For high power, you would use the appropriately rated
insulators and feed line. Please read the ARRL 'Antenna Book'.

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

Too complex to explain here. Please read the ARRL 'Antenna Book'.

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

Antennas are 'bilateral'; that is, if they are directional for transmit,
the same characteristics apply for receive. Please read the ARRL
'Antenna Book'.

Do receive antennas have power ratings?

No.


The above answers are *very* basic and don't even scratch the surface of
antenna design. The questions you ask above would require a book to
answer properly. Oh, did I mention that you should read the ARRL
'Antenna Book'? If not that, at least read the antenna chapters in the
ARRL 'Amateur Radio Handbook for Radio Communications'. (www.arrl.org)

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

Dear Vijay,
Please consult the ARRL ANTENNA HANDBOOK...
They will answer all your questions...

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.

Terry

In article
,
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