QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/digests/antenna/940259.txt

Internet Message Format | 1994-11-13 | 8KB

Date: Sat, 13 Aug 94 04:30:15 PDT
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Subject: Ham-Ant Digest V94 #259
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Ham-Ant Digest              Sat, 13 Aug 94       Volume 94 : Issue  259

Today's Topics:
                        Galvanized wire at RF?
                    Radio signalling under water?
               Should feedline lenght change the VSWR?
                      SWR calculation needed....

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We trust that readers are intelligent enough to realize that all text
herein consists of personal comments and does not represent the official
policies or positions of any party.  Your mileage may vary.  So there.
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Date: 10 Aug 1994 21:10:54 -0700
From: news.sprintlink.net!news.world.net!news.teleport.com!news.teleport.com![e-mail]
Subject: Galvanized wire at RF?
To: [e-mail]

I'm going to erect a temporary rhombic (440 feet/leg) on a cousin's farm
for an upcoming contest operation.  he's got miles of galvanized electric
fence wire spooled and available.  Does anyone have any data on the
suitability of galvanized wire at 3.5 and 7 MHz?  Failing that, does
anyone have any lead on inexpensive *real* wire that can be tensioned to
provide a 440 foot span with limited droop and stretch?  All info
welcomed.  Thanks.

Bill Conwell
K2PO/7


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Date: Fri, 12 Aug 1994 18:33:23 GMT
From: lll-winken.llnl.gov!noc.near.net!ns.draper.com!news.draper.com!jwy1294a.draper.com![e-mail]
Subject: Radio signalling under water?
To: [e-mail]

In article <[e-mail]> [e-mail] (Fred McKenzie) writes:
>From: [e-mail] (Fred McKenzie)
>Subject: Re: Radio signalling under water?
>Date: Wed, 10 Aug 1994 17:12:25 -0400

>In article <[e-mail]>, [e-mail] (Alan
>Cooney) wrote:
>> I'm in need of advice and/or information on getting signals through
>> to a submerged remote vehicle......I've considered using ultrasonics,

>Alan-

>There are systems used by divers for similar applications.  They use
>ultrasonic sound as a "carrier", which is modulated as if it were a radio
>carrier.

>I think the frequency is in the order of 100 KHz to 200 KHz.  It seems
>that there is a trade-off involved in selection of a frequency.  At lower
>frequencies, the ambient sea noises (waves, whales, et cetera),
>interfere.  At higher frequencies, signal attenuation limits the range.
>There is a "window" between these, that is useable for communications.

>I suggest you ask for information at a diver supply store.  I believe
>there are at least two companies that make these systems.  (My information
>is from about 15 years back, so please forgive my faulty memory!)

>73, Fred, K4DII


Coincidentally, I just ran into an article this morning in the library:

Coates, Dr Rodney, "Underwater Acoustic Communication," Sea Technology
Magazine, July 1994, pp 41-47.

Excellent overview coverage, especially in things like relating range and
frequency, power requirements, keying formats, and what's on the market.
Readable, too.

73,
Skip, K1NKR

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Date: 12 Aug 1994 19:41:03 -0400
From: newstf01.cr1.aol.com!search01.news.aol.com![e-mail]
Subject: Should feedline lenght change the VSWR?
To: [e-mail]

   I was hoping that perhaps one of the antenna guru's could help with a
question about SWR that
I have.   I recently bought a new vertical antenna which I installed on
the roof of my house.
I is one of the center fed type of verticals that isn't supposed to need a
ground plane that
Company "G" advertizes quite widely.   After I installed it I check the
VSWR in the shack with
my VSWR bridge and found that the VSWR on most of the bands was rather
high.   In most cases
between 2 to 1 and 4 to 1.   I then went up to the base of the antenna
with my Autek RF Analyst
and measure the VSWR and found that all of the bands had a very high SWR.
 For example,
20 Meters was 4.1 at the bottom of the band and 5.0 at the top end.   75
meters was the best
with a reading of 1.6 to 1 near the top of the band.   So I called Company
"G" and spoke to
their tech support person and he told me that one should not measure the
VSWR at the antenna.
Instead it had to be measured at the end of the feed line which had to be
65 feet long and that
the VSWR would change depending on the lenght of the feed line.  I told
him that I was under
the impression that the VSWR on the feed line should not change with
lenght unless the load,
in this case the antenna were unbalanced.   He launched into a long
lecture about how this was
only true if the load presented a perfect 50 Ohm non reactive resistance
and that in the real
world of an antenna this condition could not be met and therefor the VSWR
would change as you
change the length of the feed line.   This isn't how I understood things
worked from reading
Walter Maxwell's book "Reflections" but maybe I missed something.   It
wouldn't be the first
time!   Then he went on to tell me that the VSWR on 75 meters should be
lower than 1.6 and that
the rest of the antenna wouldn't work unless the VSWR on 75 was lower.
He asked if I had
any other antennas near the vertical and I said that I had a 40 foot
frestanding tower about 70
feet away and he told me that the tower was messing up the vertical
because it was less than
one wavelenth away!   Any one have any ideas about that one??   I want to
call him back and
further our discussion but I want to make sure I have my facts straight
first.   Is what this
guy told me correct or is he just putting out a smoke screen?   Thanks for
taking the time
to read this and I will appreciate any and all feed back.

Terry   KJ7F


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Date: Thu, 11 Aug 1994 20:23:38 GMT
From: ncrgw2.ncr.com!ncrhub2!ranger!cn2935.DaytonOH.NCR.COM![e-mail]
Subject: SWR calculation needed....
To: [e-mail]

In article <[e-mail]> [e-mail] (Tom Bruhns) writes:
>From: [e-mail] (Tom Bruhns)
>Subject: Re: SWR calculation needed....
>Date: Thu, 11 Aug 1994 16:35:36 GMT

>Brian Ellse ([e-mail]) wrote:

>: Greetings to one and all,

>: Can somebody please tell me the formula for calculating SWR when given only
>: the forward and reflected power in watts.

>:             i.e Bird Inline reads 15w forward and 6w reverse. SWR=?

[ derivation of the formula deleted, resulting in: ]
>   SWR =  (sqrt(fwdpwr) + sqrt(rvrspwr)) / (sqrt(fwdpwr) - sqrt(rvrspwr))

>(The sqrt(|Zo|) terms cancelled out between numerator and denominator.)

>It's useful to draw a graph of SWR versus reverse/fwd power and tape it
>to the back of the Bird.  It's not too hard to put the above formula
>into a spreadsheet and use it to draw the graph for you.  The square root
>causes some interesting effects:  the reverse/fwd power ratio to get to
>1.05:1 SWR, for example, is about .0006 -- pretty hard to read accurately!
>The example above, 6w reverse and 15 fwd, is about 4.44:1 SWR.

An awfully handy fact in the real world is that 10% reflected power (ie, 10
watts forward and 1 watt reflected) equals 2:1 SWR (well, actually, 1.93:1).
Since many of us figure that a SWR of 2:1 or less is a reasonable match (at
least in most cases), checking for <10% reflected power is a simple go/no-go
antenna system test.

John   AG9V
[e-mail]

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End of Ham-Ant Digest V94 #259
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