This has been discussed recently so there may be something in the
reflector archive.
Space loss formula are for far-field situations:
Pn = 32.4 + 20Log(F) + 20Log(d), where Pn is in dBm, F in MHz, d in km
Antennas as close as you have are in the antenna near-field which has
much more complicated math which I am not going to attempt. So the
only way to determine what is safe is to measure power coming in on
the KX3 antenna when you transmit with the K3 (plus amp) on the other
antenna running whatever power you plan to use.
I know many hams do not own a lot of sophisticated test equipment so
I offer three ways to make the measurement:
1- A simple method would be to connect an ordinary power meter (SWR
meter) to the KX3 antenna and connect the other connector to a load
(no radio connected). Connect so you read forward power toward the
load. I'm going to assume your meter is capable of measuring 10w
full scale so a small movement of the indicator would show about 1%
(0.1w) indication. That level is 100mw which is about 100 times too
strong for the KX3 receiver to survive. In fact maybe the KX3 can
handle 10mw but better to be safe by using 1mw as the maximum safe
input power limit.
2- One can measure weaker signals if you could obtain use of a HP432
microwave power meter as it measures from -20 to +10 dBm* (but will
burn out above this range). Probably you would want the owner of the
meter to run the test since they are expensive and delicate. *0.001
to 10mw; attenuators should be used initially like, below.
3- If you have access to some coaxial attenuators you could place
them inline with the antenna and your receiver and see what s-meter
reading results. I would start with at least 40-dB for safety for
your radio (KX3 in your case). if s-meter readings are too low then
remove attenuation in 10-dB steps until you see full scale reading of
the s-meter. At this point if you still have any attenuation in the
line then the power coming into your antenna is too strong for the radio.
For reference my KX3 reads S9+50 with a 0 dBm (1mw) signal from my
XG3 on 20-MHz (with PRE off).
I have a similar situation with 80/40m inverted-V hung on the same
tower as my triband yagi with apex about 12 feet below the yagi. I
have a 6m yagi at 30-feet below and often monitor 6m while using my
K3 on HF bands (that seems to be OK but not running the K3+125w amp
to the other 6m yagi 50-feet horizontally distant).
------------------------
QUESTION 1: The KX3 has been using an inverted V wire antenna. At its
closest point to a vertical antenna for our K3 they are about 30 feet apart.
Is this too close? How far away would be far enough if K3 operator were
using 450 watts? I do recall one morning when I used K3 while Larry had
KX3 on. I guess I will never do that again, but I would like to know what
distance is safe. We have a neighbor a block away who runs high power.
THANKS to anyone who can opine.
73,
Julie KT4JR
73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag business:
[email protected]
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