In article ,
Bob Miller wrote:
On Sun, 12 Mar 2006 22:28:19 GMT, Telamon
wrote:
In article ,
wrote:
On 12 Mar 2006 01:08:39 -0800, wrote:
Hey, you asked for opinions in your first post
of the thread. If you don't like my opinions, thats
too bad. An antenna is an antenna is an antenna.
Do you think when I switch from a SWL to a ham band,
I need to switch antenna types? LOL...You guys kill me..
MK
Well how can your receiver work with an antenna made for transmit 8-}
Basically (theoretically) an antenna made for transmit is just as
suitable for receive. This is called reciprocity.
This concept falls apart two ways in practicality:
1. The transmit situation has to handle power the receive situation does
not so for transmit the antenna elements need to be "beefier."
Most transmitting antennas use wire about the same size as what's
found in receiving-only antennas, 14 or 16 guage, maybe 12 guage for
full legal power.
Ever calculate antenna resistance at HF for those gauges? What is the
expected loss for some band you worked on?
What is legal power for hams on HF? 1500 watts? And is that continuous
power or PEP?
2. A less than full size antenna made resonant may work very well for
transmit but for receive not as well. Less than full size for receive
lowers the antenna efficiency.
I seriously doubt you could hear the difference between a full size
antenna at frequency, and one slightly shorter for space
considerations.
On receive the antenna efficiency is related to is size by means of
radiation resistance. Antenna efficiency is directly dependent on the
combination of radiation and antenna element resistance (DC+AC).
This affects the received signal power just like it does for transmit
but like I said on transmit you can use materials with higher dielectric
constants and reactive components to launch an EM wave efficiently.
However, on receive you can't affect the environment around the antenna
in the same fashion as the antenna itself therefor what I stated stands
unless you can refute it.
The key here in the non-reciprocity of antenna performance is the fact
that "you can't affect the environment around the antenna in the same
fashion as the antenna itself." The environment around the antenna has
an impedance value not affected by the antenna but antenna size directly
correlates to radiation resistance.
Think about it. If you were right then everyone would be using a whip
antenna. Why bother to build full size antennas?
--
Telamon
Ventura, California