QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/digests/antenna/940051.txt

Internet Message Format | 1994-11-13 | 17KB

Date: Tue,  1 Mar 94 04:30:14 PST
From: Ham-Ant Mailing List and Newsgroup <[e-mail]>
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Subject: Ham-Ant Digest V94 #51
To: Ham-Ant


Ham-Ant Digest              Tue,  1 Mar 94       Volume 94 : Issue   51

Today's Topics:
                            160 M on G5RV
                      Discone Design Parameters
                    INMARSAT azimuth,angle,antenna
                        Mobile Antenna Tuners
     New HF Propagation Analysis - Supports MinuiNec/ElNec output

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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.
----------------------------------------------------------------------

Date: Sun, 27 Feb 1994 13:59:52 GMT
From: ihnp4.ucsd.edu!swrinde!cs.utexas.edu!math.ohio-state.edu!darwin.sura.net!perot.mtsu.edu!raider!theporch!jackatak![e-mail]
Subject: 160 M on G5RV
To: [e-mail]

[e-mail] (Cecil Moore) writes:
> Ed Engel ([e-mail]) wrote:
> : regular
> : Keywords: G5RV
> :
> : Has anyone figured out how much wire you need to add to a regular
> : G5RV to tune it to 160 M? Mine is 102' with the 30' of 450 ohm
> : ladder line and I normally run it on 75 and 40 M through a MFJ-949D
> : tuner.  73 de Ed Engel   N7UQZ
>
> Antennas West G5RV App note says 204 ft total center-fed with 64 ft
> of 450 ohm ladder-line. I suggest 450 ohm ladder-line all the way
> to a balanced antenna tuner.

Cecil is SPOT-ON here. The *BALANCED* tuner with 450 ohm ladderline
from antenna feedpoint to tuner will work all bands 160-10M quite
well, and the conjugate match of the system will get a nice signal
out.

Remember, the original design criteria of the G5RV was a NO-tuner
system for several bands... Much of that was acheived through loss in
the coaxial feedlines used.... (See Gary Coffman's -- and others --
posts about the fleamarket coax...)

If you have a tuner anyway, run ladderline to the tuner and enjoy a
good signal.
73,
Jack, W4PPT/Mobile (75M SSB 2-letter WAS #1657 -- all from the mobile! ;^)

+--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--+
| Jack GF Hill        |Voice: [phone] -             Ham Call: W4PPT |
| P. O. Box 1685      |Modem: [phone] -  Bicycling and SCUBA Diving |
| Brentwood, TN  37024|Fax:   [phone] -          Life Member - ARRL |
| [e-mail]  -   "Plus ca change, plus c'est la meme chose" |
+--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--+

------------------------------

Date: 27 Feb 1994 16:02:58 GMT
From: ihnp4.ucsd.edu!swrinde!gatech!concert!balsam!zeno![e-mail]
Subject: Discone Design Parameters
To: [e-mail]

In article <[e-mail]> [e-mail] (Dexter Wm. Francis) writes:
> ... interesting stuff deleted for the sake of brevity ...
>
>So: Make the included angle of the drooping radials 60 degrees.
>    The low cutoff wavelength is six times the diameter of the disk.
>    The radials are 1.428 times the diameter of the disk.
>
>Enjoy
>
>-df

Many commercial discones incorporate a vertical element designed to increase
the 10:1 bandwith to extend from, say 50 - 1300 MHz.  Does anyone know how
this is done.  How long is the element and where does it connect - to the
disk or to the cone?

Regards

-Chuck



--
Chuck Bennett                    [e-mail]
Department of Physics                [phone]
Univeristy of North Carolina - Asheville
Asheville, NC 28804

------------------------------

Date: 27 Feb 1994 15:28:11 GMT
From: ihnp4.ucsd.edu!swrinde!gatech!concert!news.duke.edu!soc4.acpub.duke.edu![e-mail]
Subject: INMARSAT azimuth,angle,antenna
To: [e-mail]

I would like to try to listen to the Armed Forces Radio Television Service (AFRTS)
on 1537 MHz on the INMARSAT Atlantic Ocean-West or East.

Could someone with a satellite prediction program tell me the azimuth and angle
to these satellites from Durham NC? The satellites are geostationary at 15.5 and
55 West longitude.

I am tenatively planning to use two 3 wavelength rhombics spaced 1/2 wavelength
apart as my antenna.  Is this a good choice? Would it be better to mount the
rhombics at 90 degrees to each other for cross polarization? What would the
600 ohm rhombic antenna impedance change to at the feedpoint in either case?

I plan to use 300 ohm TV twin lead from the antenna to a TV balun to the 50 ohm
BNC of my AR-3000 scanner.  Will I need to modify the TV balun?  Do I need a
1/4 wave matching stub at the antenna?

Does any antenna company sell a modestly priced antenna for this service?

------------------------------

Date: Sun, 27 Feb 1994 13:16:52 GMT
From: ihnp4.ucsd.edu!agate!howland.reston.ans.net!darwin.sura.net!perot.mtsu.edu!raider!theporch!jackatak![e-mail]
Subject: Mobile Antenna Tuners
To: [e-mail]

Cecil Moore <[e-mail]> writes:

> One wrote me a letter and said, "For best results your HF mobile
> antenna must be resonant on your transmitting frequency."
>
> Maxwell, author of "Reflections" tells us that the radiator of an
> antenna system need not be a self-resonant length for maximum resonant
> current flow...
Correct so far, but there is more than just what Maxwell wrote...

> transmission line and the antenna does not prevent the antenna from
> absorbing all the power available at the junction.  If ten feet of
> coax at least the size of RG-8 is used in mobile HF installations,
                    ^^^^
WHOA!!! The *size* of the coax has NOTHING to do with the loss and
re-reflection back to the antenna. NOTHING! If you use 2" diameter
JUNK feedline with 10% braid coverage, what you say further is simply
not going to happen... the physical size is nowhere near as important
as the loss characteristic of the feedline, which is NOT directly
related to diameter.

For example, I am using heliflex (Andrew Cable) in my mobile
installation, and have adjusted the antenna for self-resonance at my
frequencies of choice...with a matching coil at the base. While I can
indeed increase the bandwidth by inserting a tuner, what is happening
at the antenna, and let us NOT try looking at 17-10 meters where the
physical size of the antenna approaches the unloaded size of a 1/4
wave; instead, let's look at worst (well, penultimate worst ;^) case:
75 meters...where the antenna needs to be 67 feet long to approach a
quarter wave, and physically be only about 12 feet tall. This case is
far more likely to fall into the statement you first presented: the
antenna will perform *best* (emphasis added) when self-resonant.

I run a Texas BugCatcher (no relation, just a very satisfied customer,
often accused of running a kilowatt from an RV hooked up to a "fixed"
antenna) that has been tuned and trimmed to work best at ONE place,
with a bandwidth of ~20KHz. I tried a tuner, and while the bandwidth
did increase, the performance of the antenna fell off, measureably.

Some of that may well have been the nature of the tuner, since it was
following more or less the utlimate transmatch design and provided the
possibility of imaginary solutions to the three unknowns-two equations
algebra of the tuner circuit. Just having the tuner in-line reduced my
signal substantially. There is NOTHING wrong with the unit, but the
properly tuned mobile system will only gain bandwidth at the expense
of signal efficiency, most likely because of the heavily loaded nature
of the antenna.

Your results on 10-17M would tend to bear that out, although a
ham-stick design antenna is a loaded design as well. I suspect that
the nature of those bands and the propogation characteristics have as
much to do with your observed results as the theory you quote.

I, too, have read and studied Maxwell's book. I do not find the
material therin to be at conflict with what you say, save the loaded
nature of both loads (mine at 75M and yours at 10M) doesn't jibe with
what I recall Maxwell saying... I believe he was addressing a
non-resonant *and* non-loaded antenna system, not the massive loading
one must apply, particularly at the really low frequencies, to make
the very short antenna, radiation resistance ~3 ohms, function
properly.

There are many solutions to the retuning for "resonant" coverage from
a mobile, some involving rather ingenious use of cordless screwdriver
motors to raise and lower loading coils and whips. These work
acceptably (for most people) but for DXCC or WAS on 75 meters from a
mobile, a really well-design and properly tuned antenna will perform
better than the non-resonant antenna with a tuner for increased
bandwidth.

I do not like getting out in the rain to swap taps, but when I do, the
antenna performs better than with a tuner -- I did A/B tests with a
pair of switching relays that allowed the tuner to be in-line or
bypassed, just to see...and my signal was reported back to me as
noticeably stronger without the tuner in line.

I do not see that as conflicting with what you say, but as being
explained by the properties of the tuner, the effect of a tuner in
line with a tuned system, where the tuner is NOT part of the tuning
process, but must be adjusted for minimum impact.

I believe, for mobile operations, the statement about self-resonance
working best is true. Your system may well work very satisfactorily,
but I do not believe the actual radiated energy to be the same as if
your antenna system (without the tuner) were tuned to a resonant
load.

> Once the antenna tuner is adjusted properly, the HF
> mobile antenna is FORCED into resonance.  I use a 10m Hamstick plus
> a mobile antenna tuner to cover all bands from 17m to 10m.
The antenna is a shortened, loaded design. And, the characteristics of
10, 12, 15, 17 are such that when the band is open a damp string will
radiate and make contacts... When the antenna is foreshortened to ~20%
of a 1/4 wave, however, the amount of loading is importaqnt to the
antenna's ability to absorb and radiate power.

> The antenna expert wrote, "Any instruction book on antenna tuners will
> state: The antenna tuner used between the rig and the antenna system is
> to achieve maximum performance from the transceiver..."
He is WRONG. Maxwell's book quite clearly states that in a properly
designed system, the tuner will re-reflect the power back to the
antenna load where it is "eventually" absorbed and radiated...the only
additional loss being the added feedline loss for each roundtrip along
the feedline.

This latter statement, that the additional loss, due to SWR alone, is
only that loss due to the feedline loss, and is why the charts so
clearly show that additional loss due to SWR is so small a fraction
of overall feedline loss.

> I think the antenna tuner is used between the rig and the antenna system
> to achieve MAXIMUM RADIATION FROM THE ANTENNA.
I think, if you were to stop right there, you would be correct. The
tuner, in your installation, allows the antenna to radiate as much
power as possible on the frequencies you are useing the tuner to make
the antenna work on... Sheesh! That didn't come out real well... The
tuner is permitting you to radiate power on bands the antenna was
never design to operate on by providing a conjugate match of the
system. However, the fact remains, your antenna will not perform as
well, say on 17M as a FULL 1/4 wave antenna would.

Materials have an influence, and the resistance of the wire in the
ham-stik coils and the Q of the loading are significant factors.

> As Maxwell says,  "My Transmatch really does tune my antenna".
Yep, but a closer to full-sized antenna, with Hi-Q coils will still
outperform the ham-stik...tuner or no...
73 ES TNX FER THINKING
Jack, W4PPT/Mobile (75M SSB 2-letter WAS #1657 -- all from the mobile! ;^)

+--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--+
| Jack GF Hill        |Voice: [phone] -             Ham Call: W4PPT |
| P. O. Box 1685      |Modem: [phone] -  Bicycling and SCUBA Diving |
| Brentwood, TN  37024|Fax:   [phone] -          Life Member - ARRL |
| [e-mail]  -   "Plus ca change, plus c'est la meme chose" |
+--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--+

------------------------------

Date: 27 Feb 1994 15:34:47 -0500
From: ihnp4.ucsd.edu!agate!usenet.ins.cwru.edu!nigel.msen.com!yale.edu!noc.near.net!news.delphi.com!news.delphi.com![e-mail]
Subject: New HF Propagation Analysis - Supports MinuiNec/ElNec output
To: [e-mail]

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------------------------------

End of Ham-Ant Digest V94 #51
******************************