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Old October 2nd 03, 05:43 AM
Nick C
 
Posts: n/a
Default Yagi / Beam antenna theory question...

Sorry for the dumb question, but the ARRL antenna books just confuse me
sometimes. I seem to get more questions than I get answers to.

Question: In an HF beam antenna, such as a Yagi... the forward gain
measurement... is this in terms of the output gain (ERP) or does this also
mean the receiving gain as well?

The reason why I'm asking: I'm trying to come up with the ideal SWL
antenna. I currently have a 75ft wire antenna using the RS kit. This does
a pretty good job of pulling in signals. I'm just wondering if there's a
better option.... such as a multi-band HF beam antenna..?
So what antenna would give me the best "gain" in terms of receiving power.
I'm doing no transmitting here...


Thanks

--
-Nick
-----------------------------
"The metric system did not really catch on in the United States, unless you
count the increasing popularity of the nine-millimeter bullet."



  #2   Report Post  
Old October 2nd 03, 08:29 AM
Roy Lewallen
 
Posts: n/a
Default

The gain (that is, the strength of a signal sent or received in a given
direction compared to a reference antenna) is the same for transmitting
and receiving. However, gain is often the least important criterion for
a receiving antenna. The reasons why are summarized in a posting I made
just a few days ago. Go to http://www.groups.google.com, choose
"Advanced search", and locate the posting I made on Sept. 20 on this
newgroup, on the thread "Ten-tec vee beam". I and others have also made
similar postings in the past on the topic, which you should be able to
locate by searching for a few key words or phrases such as "receiving
antennas".

Roy Lewallen, W7EL

Nick C wrote:
Sorry for the dumb question, but the ARRL antenna books just confuse me
sometimes. I seem to get more questions than I get answers to.

Question: In an HF beam antenna, such as a Yagi... the forward gain
measurement... is this in terms of the output gain (ERP) or does this also
mean the receiving gain as well?

The reason why I'm asking: I'm trying to come up with the ideal SWL
antenna. I currently have a 75ft wire antenna using the RS kit. This does
a pretty good job of pulling in signals. I'm just wondering if there's a
better option.... such as a multi-band HF beam antenna..?
So what antenna would give me the best "gain" in terms of receiving power.
I'm doing no transmitting here...


Thanks


  #3   Report Post  
Old October 2nd 03, 11:39 AM
Nick C
 
Posts: n/a
Default

Roy,

Thanks for the update. I just read those postings. That V antennas looks
similiar to what I have, except I feed at the end of my wire. Also, my wire
isn't terminated and is not grounded. (Whenever I ground my wire, I get a
severe loss of signal...?)

Anyway, after seeing the price of their antenna, I quickly decided that mine
isn't so bad after all...

A couple of quick questions:

1. It sounds like, at least for my suburb environment, I'm probably not
going to improve my reception by having a yagi antenna. So, as you said,
it's better to reduce the noise coming into the receiver. I get a lot of
QRM from non-atmospheric noise... although it seems like recently, the
atmospheric noise has been really bad. (Even WWV doesn't come in without
extra noise.)

2. I've never measured the SWR of my antenna. Will this have any affect on
my signal reception? I was under the impression that SWR was really only
valid if you're transmitting energy..?

3. Really dumb question here. If a branch from a tree is touching my wire,
then is my antenna effectively cut in length... from that point to my
feedpoint? My construction of the antenna was done rather quickly, so if
this is the case, then I'll need to relocate the wire.

(FYI, currently, I have rougly a 100ft length of wire, which actually is in
a V-shape... almost the shape of the house. The wire itself is a good 30ft
off the ground... which should be 'above' the QRM levels of the surrounding
houses. My endpoint is a simple balun which converts the wire Z to 50ohms
.... and then I use low-loss coax to my rig.)

I'm just looking to 'improve' my design if that's possible... and definitely
increase my S/N ratio as you suggested.


Thanks for your time.
-Nick

"Roy Lewallen" wrote in message
...
The gain (that is, the strength of a signal sent or received in a given
direction compared to a reference antenna) is the same for transmitting
and receiving. However, gain is often the least important criterion for
a receiving antenna. The reasons why are summarized in a posting I made
just a few days ago. Go to http://www.groups.google.com, choose
"Advanced search", and locate the posting I made on Sept. 20 on this
newgroup, on the thread "Ten-tec vee beam". I and others have also made
similar postings in the past on the topic, which you should be able to
locate by searching for a few key words or phrases such as "receiving
antennas".

Roy Lewallen, W7EL

Nick C wrote:
Sorry for the dumb question, but the ARRL antenna books just confuse me
sometimes. I seem to get more questions than I get answers to.

Question: In an HF beam antenna, such as a Yagi... the forward gain
measurement... is this in terms of the output gain (ERP) or does this

also
mean the receiving gain as well?

The reason why I'm asking: I'm trying to come up with the ideal SWL
antenna. I currently have a 75ft wire antenna using the RS kit. This

does
a pretty good job of pulling in signals. I'm just wondering if there's

a
better option.... such as a multi-band HF beam antenna..?
So what antenna would give me the best "gain" in terms of receiving

power.
I'm doing no transmitting here...


Thanks




  #4   Report Post  
Old October 2nd 03, 06:46 PM
Roger Halstead
 
Posts: n/a
Default

On Wed, 1 Oct 2003 21:43:23 -0700, "Nick C" wrote:

Sorry for the dumb question, but the ARRL antenna books just confuse me
sometimes. I seem to get more questions than I get answers to.

Question: In an HF beam antenna, such as a Yagi... the forward gain
measurement... is this in terms of the output gain (ERP) or does this also
mean the receiving gain as well?


Yes, it works in both transmit and receive.


The reason why I'm asking: I'm trying to come up with the ideal SWL
antenna. I currently have a 75ft wire antenna using the RS kit. This does


If there were an "ideal" antenna we'd all be using it. Much depends
on the space available, (length, area, and height) and how wide a
frequency range (or frequency segments) you wish to cover.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

a pretty good job of pulling in signals. I'm just wondering if there's a
better option.... such as a multi-band HF beam antenna..?
So what antenna would give me the best "gain" in terms of receiving power.
I'm doing no transmitting here...


Thanks


  #5   Report Post  
Old October 2nd 03, 09:48 PM
Roy Lewallen
 
Posts: n/a
Default

Nick C wrote:
Roy,

Thanks for the update. I just read those postings. That V antennas looks
similiar to what I have, except I feed at the end of my wire. Also, my wire
isn't terminated and is not grounded. (Whenever I ground my wire, I get a
severe loss of signal...?)

Anyway, after seeing the price of their antenna, I quickly decided that mine
isn't so bad after all...

A couple of quick questions:

1. It sounds like, at least for my suburb environment, I'm probably not
going to improve my reception by having a yagi antenna. So, as you said,
it's better to reduce the noise coming into the receiver. I get a lot of
QRM from non-atmospheric noise... although it seems like recently, the
atmospheric noise has been really bad. (Even WWV doesn't come in without
extra noise.)


It's hard to say. If the noise is primarily from one direction, AND it's
not the direction of signals you want to listen to, AND the Yagi has
reasonable rejection of signals coming from the noise source (which
might be at an elevation angle where Yagi rejection isn't very good),
then a Yagi would help. Otherwise, you're right, it wouldn't. I agree
that the odds are against it helping much.

2. I've never measured the SWR of my antenna. Will this have any affect on
my signal reception? I was under the impression that SWR was really only
valid if you're transmitting energy..?

No, the SWR won't affect your reception. If the noise level drops when
you disconnect your antenna, then improving the SWR won't help a bit.

3. Really dumb question here. If a branch from a tree is touching my wire,
then is my antenna effectively cut in length... from that point to my
feedpoint? My construction of the antenna was done rather quickly, so if
this is the case, then I'll need to relocate the wire.

A dry tree touching the wire probably won't have any effect. If the tree
is wet, its effect on the antenna depends on the frequency and where it
touches the antenna. If it touches at a high-impedance point (near an
integral number of half wavelengths from an end), it'll effectively
connect the tree to the antenna. If it's at a low impedance point (near
an odd number of quarter wavelengths from an end), it won't have an
appreciable effect.

(FYI, currently, I have rougly a 100ft length of wire, which actually is in
a V-shape... almost the shape of the house. The wire itself is a good 30ft
off the ground... which should be 'above' the QRM levels of the surrounding
houses. My endpoint is a simple balun which converts the wire Z to 50ohms
... and then I use low-loss coax to my rig.)

I'm just looking to 'improve' my design if that's possible... and definitely
increase my S/N ratio as you suggested.


It sounds like you're end feeding the antenna. If so, even with the
balun, your feedline is almost certainly a major part of the antenna.
You might make a substantial improvement by center feeding your antenna,
with a balun at the feedpoint. The balun won't be effective at all
frequencies, but even where it isn't, your feedline will be much less
part of the antenna than when it's end fed. It's quite possible that the
feedline is picking up the local noise more strongly than it's getting
distant signals, because it's closer to potential noise sources and
because it's probably more vertical than the antenna itself. So reducing
feedline pickup could improve your signal-to-noise ratio. I recommend
giving center feeding a try.

Ironically, you might have found that the Yagi makes quite an
improvement -- but not because of its gain. People replacing something
like you've got with a properly fed Yagi that's high and in the clear
often find a big improvement, and mistakenly attribute it to the Yagi's
gain. What they don't realize is that if they'd put a dipole up in the
same place with the same care, they'd see just about the same improvement.


Thanks for your time.
-Nick


Hope this is helpful. If it is, it's time well spent.

Roy Lewallen, W7EL



  #6   Report Post  
Old October 3rd 03, 01:47 AM
Nick C
 
Posts: n/a
Default

Roy,

Yes, thanks. That confirms most of what I already knew.

About that tree question though... that one threw me. I thought for sure,
if the tree was wet (which will be about 99% of the time from now on)...
would 'remove' the energy from any signal hitting the wire..? The reception
quality with aspect to it's wavelength makes sense, and I understand that.

And yes, I have thought about using a dipole, but you clarified one point --
that it's not multiband. Although there are a few vendors who sell these
SWL type antennas. Most are end-fed, but there are a few that are
center-fed. Center feeding would be a lot harder for me to set up -- a lot
more coax cable to run to my rig!

PS. My coax is actually pretty horizontal. I have to fling it over some
other objects, so it actually stays about 90% horizontal. My
computers/tv/video don't seem to affect it too much with coax. My old set
up -- with just plain incoming wire... was really bad with QRM -- as
expected.


Thanks
-Nick
n7zya


"Roy Lewallen" wrote in message
...
Nick C wrote:
Roy,

Thanks for the update. I just read those postings. That V antennas

looks
similiar to what I have, except I feed at the end of my wire. Also, my

wire
isn't terminated and is not grounded. (Whenever I ground my wire, I get

a
severe loss of signal...?)

Anyway, after seeing the price of their antenna, I quickly decided that

mine
isn't so bad after all...

A couple of quick questions:

1. It sounds like, at least for my suburb environment, I'm probably

not
going to improve my reception by having a yagi antenna. So, as you

said,
it's better to reduce the noise coming into the receiver. I get a lot

of
QRM from non-atmospheric noise... although it seems like recently, the
atmospheric noise has been really bad. (Even WWV doesn't come in

without
extra noise.)


It's hard to say. If the noise is primarily from one direction, AND it's
not the direction of signals you want to listen to, AND the Yagi has
reasonable rejection of signals coming from the noise source (which
might be at an elevation angle where Yagi rejection isn't very good),
then a Yagi would help. Otherwise, you're right, it wouldn't. I agree
that the odds are against it helping much.

2. I've never measured the SWR of my antenna. Will this have any

affect on
my signal reception? I was under the impression that SWR was really

only
valid if you're transmitting energy..?

No, the SWR won't affect your reception. If the noise level drops when
you disconnect your antenna, then improving the SWR won't help a bit.

3. Really dumb question here. If a branch from a tree is touching my

wire,
then is my antenna effectively cut in length... from that point to my
feedpoint? My construction of the antenna was done rather quickly, so

if
this is the case, then I'll need to relocate the wire.

A dry tree touching the wire probably won't have any effect. If the tree
is wet, its effect on the antenna depends on the frequency and where it
touches the antenna. If it touches at a high-impedance point (near an
integral number of half wavelengths from an end), it'll effectively
connect the tree to the antenna. If it's at a low impedance point (near
an odd number of quarter wavelengths from an end), it won't have an
appreciable effect.

(FYI, currently, I have rougly a 100ft length of wire, which actually is

in
a V-shape... almost the shape of the house. The wire itself is a good

30ft
off the ground... which should be 'above' the QRM levels of the

surrounding
houses. My endpoint is a simple balun which converts the wire Z to

50ohms
... and then I use low-loss coax to my rig.)

I'm just looking to 'improve' my design if that's possible... and

definitely
increase my S/N ratio as you suggested.


It sounds like you're end feeding the antenna. If so, even with the
balun, your feedline is almost certainly a major part of the antenna.
You might make a substantial improvement by center feeding your antenna,
with a balun at the feedpoint. The balun won't be effective at all
frequencies, but even where it isn't, your feedline will be much less
part of the antenna than when it's end fed. It's quite possible that the
feedline is picking up the local noise more strongly than it's getting
distant signals, because it's closer to potential noise sources and
because it's probably more vertical than the antenna itself. So reducing
feedline pickup could improve your signal-to-noise ratio. I recommend
giving center feeding a try.

Ironically, you might have found that the Yagi makes quite an
improvement -- but not because of its gain. People replacing something
like you've got with a properly fed Yagi that's high and in the clear
often find a big improvement, and mistakenly attribute it to the Yagi's
gain. What they don't realize is that if they'd put a dipole up in the
same place with the same care, they'd see just about the same improvement.


Thanks for your time.
-Nick


Hope this is helpful. If it is, it's time well spent.

Roy Lewallen, W7EL



  #7   Report Post  
Old October 3rd 03, 04:40 AM
Tarmo Tammaru
 
Posts: n/a
Default


"Nick C" wrote in message
...
.............About that tree question though... that one threw me. I

thought for sure,
if the tree was wet (which will be about 99% of the time from now on)...
would 'remove' the energy from any signal hitting the wire..? The

reception
quality with aspect to it's wavelength makes sense, and I understand

that.............................................. .................

Nick,

My 75 meter antenna goes through several tree branches. Never saw any change
in SWR, wet or dry.

Tam/WB2TT


  #8   Report Post  
Old October 3rd 03, 05:30 PM
Richard Clark
 
Posts: n/a
Default

On Thu, 2 Oct 2003 23:40:31 -0400, "Tarmo Tammaru"
wrote:
My 75 meter antenna goes through several tree branches. Never saw any change
in SWR, wet or dry.


Hi Tam,

Never saw? Could he prune?

73's
Richard Clark, KB7QHC
  #9   Report Post  
Old October 3rd 03, 09:09 PM
Tarmo Tammaru
 
Posts: n/a
Default


"Richard Clark" wrote in message
...
On Thu, 2 Oct 2003 23:40:31 -0400, "Tarmo Tammaru"
wrote:
My 75 meter antenna goes through several tree branches. Never saw any

change
in SWR, wet or dry.


Hi Tam,

Never saw? Could he prune?

73's
Richard Clark, KB7QHC


Hi Richard,

The antenna is 70 feet up, as are the branches. I am beyond tree climbing
age. Still good with the bow & arrow, though.

Tam/WB2TT


  #10   Report Post  
Old October 4th 03, 09:21 AM
Roger Halstead
 
Posts: n/a
Default

On Thu, 2 Oct 2003 23:40:31 -0400, "Tarmo Tammaru"
wrote:


"Nick C" wrote in message
...
.............About that tree question though... that one threw me. I

thought for sure,
if the tree was wet (which will be about 99% of the time from now on)...
would 'remove' the energy from any signal hitting the wire..? The

reception
quality with aspect to it's wavelength makes sense, and I understand

that............................................. ..................

Nick,

My 75 meter antenna goes through several tree branches. Never saw any change
in SWR, wet or dry.


I have two 80 meter slopers. They are identical except for position.
The were even cut together and use identical (hopefully) baluns.

The tuning is quite different when I move away from resonance. One
runs through trees and the other is over an open yard.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

Tam/WB2TT




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