2 Antenna Queries
Mike.Tope at Sciatl.COM
Mike.Tope at Sciatl.COM
Fri Nov 17 15:55:00 EST 1995
On November 17 Steve KC2X wrote:
=20
=20
>QUESTION #1
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>When I see a calculated number for the impedance of an
>antenna (specifically, I'm thinking of the numbers generated
>by a Yagi-Uda modeling program), it generates tables of numbers=
that=20
>look something like: "41.00 + j2.25". =20
>From the software doc, I see that the "41.00" number is the
>Radiation Resistance (I'll call it "Rr").
=20
>Now, I've seen the definition of Rr, and I understand that
>the higer the Rr, the more power the antenna is radiating.
>Presuming that I have this fact correct, I then wonder this:
>if I model another antenna, and it's Rr is calculated to=20
>be: 19.29 -j8.34, does this mean that it is accepting only
>about HALF of the power of the first antenna? In other
>words, does antenna #1 have a 3dB advantage right off the
>bat, even BEFORE the gain is calculated?
=20
=20
Steve, it's important not to confuse the true radiation resistan=
ce of=20
an antenna with the real part of its input impedance. I am no ex=
pert,=20
but from what I remember, the radiation resistance is the real p=
art of=20
inverse ratio of the maximum antenna current to the voltage at t=
he=20
point where the current is maximum. A good example is a half-wav=
e=20
dipole, if you feed it at its center, the input impedance is aro=
und 73=20
ohms resistive. If you feed the same 1/2 wave dipole from the en=
d, the=20
input impedance will be some awfully large number which depends =
on the=20
thickness of the wire - probably 2000 ohms resistive or somethin=
g like=20
that. Now just because you feed it at the end, doesn't mean that=
the=20
gain goes up by 10*log(2000/73). Assuming you achieve the same c=
urrent=20
distribution with the end feed as center feed, and you shove the=
same=20
amount of power into the antenna thru proper matching, the radia=
ted=20
power will be the same. So what of the radiation resistance? Act=
ually,=20
I think (it's been a while) it's going to be 73 ohms in either c=
ase.=20
=20
Actually, glancing at Kraus - "Antennas, 2nd Ed." it looks like =
a good=20
definition would be the total power radiated divided by the squa=
re of=20
the current maximum, although I think the precise definition may=
be=20
probably a little more esoteric than that. =20
=20
=20
>QUESTION #2
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>This question concerns how to model LARGE vertical stacks of=
=20
>antennas.
=20
>I have read somewhere in the dozen or so antenna books I
>have laying around here that there is a sigificant difference i=
n the=20
>patterns between, say, a four-high stack of Yagis which are all=
fed=20
>in phase with equal lengths of=20
>feedline, and the same four-high stack which is broken up into,=
say,=20
>the top two antennas fed in phase, and the bottom two antennas =
fed in=20
>phase, and then the two feedlines which
>result in those phasings being fed in phase.
=20
>As I said, I've seen reference to that fact that there's a
>difference, but I've never seen any examples of what those
>differences might look like.
=20
>When stacking antennas, the Yagi modeling program I use asks fo=
r the=20
>height of each antenna (of course) but also the
>current in %, and the phase relative to "antenna #1". It=20
>seems to me that this is the area of the program that would
>be manipulated to come up with the "stack of stacks" answer,
>but I have not got a clue as to what to tell it.
=20
>I should have studied harder in school, and been an
>engineer.
=20
>Thanks for your help.
=20
>73,
>Steve KC2X
>ssacco at mcimail.com
=20
>** I love the smell of hot amplifiers in the morning!=20
> It smells like....VICTORY! **
=20
If there were no mutual coupling, i.e. your yagis were ideal bla=
ck=20
boxes, you wouldn't have a problem - you could feed the stack ei=
ther=20
way with the same result. The gotcha in the whole equation will =
be the=20
mutual coupling between antennas, and between antennas and groun=
d.=20
Sounds like your modeling program accepts a certain current phas=
or for=20
each antenna, but it may not tell you how to achieve that curren=
t when=20
you hook these things together. In other words, say you want the=
m all=20
in phase with equal current. When you model it, its easy, you ju=
st=20
tell the program 1.0 @ 0=F8 for each antenna, and you get a pret=
ty=20
picture on the screen. When you climb the tower and hook those e=
qual=20
lengths of coax to the yagis, and connect them to your ideal pow=
er=20
splitter (-6dB @ 0=F8 per leg) the story changes. Because of mut=
ual=20
coupling the driving point impedance of each antenna will probab=
ly be=20
different. This means that if each antenna is fed 1 watt from th=
e=20
power divider, the feedpoint currents relative magnitudes and ph=
ases=20
will depend upon the relative magnitudes and phases of the drivi=
ng=20
point impedances. If you get lucky and each antenna turns out to=
be 50=20
+ j0 (fat chance) then reality will closely match virtual realit=
y. If=20
on the other hand, one antenna is 100 + j0 and the other is 50 +=
j0=20
(even fatter chance) because of mutual coupling, then one antenn=
a (the=20
50 ohm antenna) will be excited with twice the current of the ot=
her=20
(the 100 ohm antenna). This is all theory of course! The reality=
is=20
that you have a big tower with stacks and I have a pine tree wit=
h an=20
inverted-vee. So much for theory!
=20
I hope this helps. If not, pretend I am nuts and ignore everythi=
ng I=20
have said.
=20
73 de Mike, AD4VH
=20
P.S. I studied hard in school. Take my advice, you did the right=
=20
thing! =20
>From Al Kinnon <akinnon at wglbosgwy.wgl.com> Sat Nov 18 00:33:00 1995
From: Al Kinnon <akinnon at wglbosgwy.wgl.com> (Al Kinnon)
Date: Fri, 17 Nov 95 16:33:00 PST
Subject: Digital Voice Keyer/Sound Blaster
Message-ID: <30AD3770 at wglbosgwy.wgl.com>
Don't have the details with me but the maker of NA sells the software to
accomplish this task. it's not as comprehensive as CT with the DVP
----------
From: owner-cq-contest%tgv.com
To: cq-contest%tgv.com
Subject: Digital Voice Keyer/Sound Blaster
Date: Friday, November 17, 1995 3:56PM
Anyone out there in contest land know of sources for software which would
allow me to use my Sound Blaster board and computer hard drive as a digital
voice keyer for contesting?
73 and see you in SS this weekend.
Dale
Dale Gaudier - N4REE/1 Darien, CT
>From Pete Soper <psoper at encore.com> Fri Nov 17 21:38:49 1995
From: Pete Soper <psoper at encore.com> (Pete Soper)
Date: Fri, 17 Nov 1995 16:38:49 EST
Subject: Contest Voice Blaster summary
Message-ID: <10987.9511172138 at earl.encore.com>
gaudier at new-york.sl.slb.com (Dale Gaudier) writes:
> Anyone out there in contest land know of sources for software which would
> allow me to use my Sound Blaster board and computer hard drive as a digital
> voice keyer for contesting?
I guess this is the right time to summarize my answers to the same question
just a short while ago. I was expecting a catalog and was going to send
more details, but here's what I have so far.
There is a commercial product named "Contest Voice Blaster" (CVB) that
works with a sound card and a simple rig interface to take care of voice
keying. The vendor is:
LTA
Box 77
New Bedford, PQ 16140
Canada
216-565-9950
Here are some answers to questions I sent to K3LR, a spokesman at LTA,
once I found him:
>CVB runs as a TSR behind CT. The same CT (DVP) record and play
>commands work with CVB. A small circuit is required to interface the
>CVB with your rig. The circuit diagram is provided with the CVB
>software. It does not send callsigns (why would anyone want this?).
>The CVB is available as a TSR within any program.
(Tim is saying CVB doesn't send synthetic callsigns derived from
logged data. It of course sends any pre-recorded callsign or
anything else you want to broadcast.)
The CVB and NA (contest software from the same vendor) are discussed
on the "NA reflector", accessible via the usual "subscribe" message to:
na-user-REQUEST at ve7tcp.ampr.org (notice that this is not at
eng.pko.dec.com anymore)
There is a high quality Web interface to the mailing list archives
for this list available at:
http://ve7tcp.ampr.org/Lists/INDEX
Reading all these archives, you'll find precious little about CVB. I got a
few comments via email however. VE6LB gave CVB a rave review, writing that it
worked great at a Canadian contest site. Mike Walker wrote that the interface
to CT isn't as good as DVP.
Haven't used CVB yet, don't have connections with LTA, just "passing it along".
Thanks to VE6LB, Mike Walker, KF8TY1 for the pointers and comments and to
K3LR for the product details.
Regards
------------------------------------------------------------------------------
Pete Soper (psoper at encore.com) KS4XG 1+ 919 481 3730, 481 3868/FAX
Encore Computer Corp 901 Kildaire Farm Rd Cary, NC 27511 USA grid FM05oq
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