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Author Topic: if I hear better do I transmit better?  (Read 1238 times)
KA1J
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Posts: 169




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« on: October 31, 2012, 03:30:34 PM »

I can't have a beam nor tower where I live. The best I can do is use a 5 position remote coax switch and have 5 separate antenna attached to a common radial plate. I have roughly 40 130' radials on the ground and use the coax switch to select which antenna for which frequency. I have; 160 inv-L, 80 inv-l (mostly vertical), 40 vert, 30 vert & butternut HF9-V in the center. I can only use the 130' inv-L for 160 so that's a given. same with 80 for 80, 40 for 40, 30 for 30 & I use the Butternut for 20. On the higher bands though, I get a nice SWR on the 80 & 160 antenna, just as close as the Butternut as far as reflected power. I use the 40 to get to 15.

So here's what I'm curious about: Lets say I'm on 17 or 12, sometimes I hear better on the 160 & sometimes on the 80, I never hear better on the Butternut.  If I hear better on a given antenna, should I expect to also be heard better with it as well? Some days I find the 160 is the best to hear with & other days the 80 is the best. Doesn't seem to be confined to direction, just some days one does better than the other for Rx & I just wonder; if a wire antenna hears better, does it transmit better as well? The SWR I read is close to the same for either the 160 or 80.

Gary
KA1J
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WB6BYU
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Posts: 13244




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« Reply #1 on: October 31, 2012, 04:20:37 PM »

Maybe, but not necessarily.

I had a long contact from Oregon to Hawaii where I switched back and forth
between a beam and a long wire.  Sometimes one was better, sometimes the other,
and it varied on a 5 to 10 minute cycle.  The results were similar at the other end:
sometimes one antenna was better than the other, then it would reverse.  But
the optimum receive antenna didn't correlate directly with the transmit antenna.

We continued for nearly an hour, switching back and forth and comparing results. 
As far as I can tell, most of the difference was due to signal polarization.  The
regular fading was due to polarization rotation changing over time, and the rotation
wasn't consistent in both directions.


So, in your case, you may find that the antenna that has the strongest receive
signal often will be better on transmit, but sometimes not.  And which antenna
is better (in either direction) may change every few minutes.
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WX7G
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« Reply #2 on: October 31, 2012, 04:29:42 PM »

An antenna is reciprocal for transmit and receive. For a free-space path the antenna that receives best will transmit best.

The ionosphere is usually but not always reciprocal. So as WB6BYU says "...the antenna that has the strongest receive signal often will be better on transmit, but sometimes not." From what I've read and looking at my log (comparing signal strength reports where we both ran the same power and the report was S-8 or below) it looks like the ionosphere is reciprocal much more often than not.

So, I would transmit with the antenna that receives best.
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KA1J
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Posts: 169




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« Reply #3 on: October 31, 2012, 04:39:41 PM »

Innately I'd think the antenna I hear best on should be the one to transmit on. Sometimes the one with the best SWR is not the one I hear best with. The next day the signal strengths may be opposite. It's been puzzling me for some time.
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NK7Z
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Posts: 790


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« Reply #4 on: October 31, 2012, 06:16:33 PM »

Hi,

I tend to believe that if I hear someone better on one antenna, he/she will hear me better...  Most of the time this works...  Every now and then it does not.  But for the most part, if you hear better, the far station will hear you better...

Thanks,
Dave
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
G3TXQ
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Posts: 1515




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« Reply #5 on: November 01, 2012, 09:02:42 AM »

We shouldn't necessarily equate "hear best" with "strongest receive signal".

I have a top-loaded vertical that I use as a transmit antenna on 160m; but I nearly always switch to my 20m thru 10m mini-beam to receive - signals are very much weaker but the S/N is higher.

Steve G3TXQ
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W8JI
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« Reply #6 on: November 01, 2012, 09:26:17 AM »

if a wire antenna hears better, does it transmit better as well?

NO. Don't let anyone convince you that "hearing better" means you transmit better.

A single sharp null in the direction of noise or QRM can make an antenna "hear better", but the same antenna can have much less gain for transmitting in the desired directions. Many other things can cause this same effect, also.

As a rule if the ABSOLUTE signal strength level over is better on receive, an antenna will transmit better. That is entirely different than "hearing better".

Let me give you an example....

I live in a quiet rural location, and I can consistently night-after-night hear and copy Europeans on 160 meters better on a five foot whip antenna than on my 200-foot tall vertical.

Signals not even audible or detectable in the noise on the tall vertical, which makes it impossible to know the absolute levels, are clearly heard on a short whip in the woods. There is probably over 30 dB more transmit signal level on the tall vertical.

For receiving below VHF (sometimes higher), antenna pattern and response to noise sources generally dominate ability to copy. For transmitting, only absolute gain at the particular angle, polarization, and in the correct direction matter.

Those two cases (receive quality and transmit quality) can be, and often are, vastly different.

Here's another thing to consider....

When a system has loss, it is not always bilateral. A tuner that transforms 50 ohms to 500 ohms, if it has loss, does NOT transform 500 ohms to 50 ohms.

On receiving, the receiver end sets feedline SWR and feedline loss.  When transmitting, the antenna end sets feedline SWR and feedline loss.

I can match a 50 transmitter to a 10 ohm antenna with a series 40 ohm resistor. The transmitter will see a 1:1 SWR. If I put a 50 ohm receiver on the system, the 10 ohm antenna sees a 90 ohm load and would have a  9:1 SWR.   

What happened to reciprocity? Think about that a while. Reciprocity does not always exist in systems with loss.

73 Tom
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