QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/digests/antenna/940084.txt

Internet Message Format | 1994-11-13 | 20KB

Date: Wed, 30 Mar 94 04:30:16 PST
From: Ham-Ant Mailing List and Newsgroup <[e-mail]>
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Subject: Ham-Ant Digest V94 #84
To: Ham-Ant


Ham-Ant Digest              Wed, 30 Mar 94       Volume 94 : Issue   84

Today's Topics:
                      Building Your Own Antenna?
                CB Mag mount for 10M mobile? (4 msgs)
                          Cushcraft A4 model
                      Need advice for HF antenna
                    Need help resonating 2m dipole
                 Question about mobile antenna 40/80m
             TVRO Satellite Dishes on Ham Towers (3 msgs)
                      What length for 2m radials

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We trust that readers are intelligent enough to realize that all text
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policies or positions of any party.  Your mileage may vary.  So there.
----------------------------------------------------------------------

Date: 29 Mar 94 16:37:27 GMT
From: hp-cv!hp-pcd!hpcvsnz![e-mail]
Subject: Building Your Own Antenna?
To: [e-mail]

Nelson Fernandez Jr ([e-mail]) wrote:

: Can someone suggest the best type of antenna for scanning? I have a coilw
: whip on my portable scanner, but I would like a better antenna for when I
: scan at home.

"Best" depends on what you want to do.  If you want to receive only things
in a 2MHz band from a town 100 miles away, "best" might well be a
highly directional array up on a tower.  If you want to receive everything
from 30MHz to 1GHz in the local area, then a discone is probably a good
choice (perhaps even two of them, one for the upper portion and one for the
lower portion of the frequency range).  Guessing that the latter is more
like what you want to do, you can find plans for discones in some ham pubs.
The RSGB VHF manual comes to mind, but surely it's covered in some ARRL
pubs too.

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

Date: 28 Mar 1994 18:29:43 GMT
From: elroy.jpl.nasa.gov!sdd.hp.com!hpscit.sc.hp.com!icon!hpchase.rose.hp.com![e-mail]
Subject: CB Mag mount for 10M mobile?
To: [e-mail]

I have been thinking about trying out HF mobile operation, and I've been
looking for a good way to get started.  I have an IC-737, which ought to
work pretty well mobile.  It occured to me that I should be able to get
a CB mag mount antenna and trim it a bit to use it on 10M.  I looked at
Radio Shack and they have a couple of them, and some even mention being
usable for 10M with some trimming.  But they also had some power limitations,
like 20W max or something.  So I guess my questions are, has anyone
successfully used one of these for 10M, and also why the power limitation?
What component(s) in an antenna would restrict the power?

Chris Moore
N6IYS
[e-mail]

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

Date: Mon, 28 Mar 94 21:31:06 PST
From: ihnp4.ucsd.edu!agate!howland.reston.ans.net!news.cac.psu.edu!news.pop.psu.edu!ctc.com!news.mic.ucla.edu!unixg.ubc.ca!nntp.cs.ubc.ca!mala.bc.ca!oneb!ham![e-mail]
Subject: CB Mag mount for 10M mobile?
To: [e-mail]

[e-mail] (Chris Moore) writes:

> I have been thinking about trying out HF mobile operation, and I've been
> looking for a good way to get started.  I have an IC-737, which ought to
> work pretty well mobile.  It occured to me that I should be able to get
> a CB mag mount antenna and trim it a bit to use it on 10M.  I looked at
> Radio Shack and they have a couple of them, and some even mention being
> usable for 10M with some trimming.  But they also had some power limitations,
> like 20W max or something.  So I guess my questions are, has anyone
> successfully used one of these for 10M, and also why the power limitation?
> What component(s) in an antenna would restrict the power?
>
> Chris Moore
> N6IYS
> [e-mail]


Why bother with a CB antenna? Larsen makes a range of antennas in a
variety of mounts, including magnetic, that cover 28-30 MHz and claim to
be good for 200 watts, which ought to handle most current mobiles.

They are certainly rugged, and last well.


Robert Smits                    There is *no* idiotproof filter.
VE7EMD                          Idiots are proof against anything!
Ladysmith B.C.                       - Richard Chycoski, VE7CVS
e-mail: [e-mail]

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

Date: 29 Mar 94 04:56:22 GMT
From: dog.ee.lbl.gov!agate!howland.reston.ans.net!gatech!news-feed-2.peachnet.edu!emory!nntp.msstate.edu!olivea!sgigate.sgi.com!sgiblab!cs.uoregon.edu!reuter.cse.ogi.edu![e-mail]
Subject: CB Mag mount for 10M mobile?
To: [e-mail]

In article <2n77mn$[e-mail]>,
Chris Moore <[e-mail]> wrote:
>I have been thinking about trying out HF mobile operation, and I've
been
>looking for a good way to get started.  I have an IC-737, which ought
to
>work pretty well mobile.  It occured to me that I should be able to get
>a CB mag mount antenna and trim it a bit to use it on 10M.  I looked at
>Radio Shack and they have a couple of them, and some even mention being
>usable for 10M with some trimming.  But they also had some power
limitations,
>like 20W max or something.  So I guess my questions are, has anyone
>successfully used one of these for 10M, and also why the power
limitation?
>What component(s) in an antenna would restrict the power?
>
>Chris Moore
>N6IYS
>[e-mail]


I haven't seen that particular antenna, but the component most likely to
restrict the power handling capability of a mobile antenna is the
insulation of the loading coil.  At the 100 watt level several thousand
volts can be generated across the coil.  This can cause arcing and also
can cause heating if the coil is wound on a lossy form.

I assume the reason you are asking about mag-mounts is that you don't
want to drill holes...right?  Tsk, tsk.  Get out that drill and do it!
Survey after survey has shown that antenna holes do not affect a car's
resale value.  Personally, I think that if you're going to go mobile,
you ought to do it right, and a mag-mount for hf ain't it.  Get a Radio
Shack 102" stainless steel whip and a ball mount.  It will handle full
amateur power with essentially zero loss and maximum possible bandwidth.

And as long as I'm on the soapbox, don't stop with ten meters.  Try 40
and 20 - they're both excellent mobile bands, especially 40 because it's
open 24 hours a day to somewhere.  IMHO, either one is far superior to
10 for general all-round mobiling.

flame supressor now on...

73 es gl

Bill, W7LZP


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

Date: Tue, 29 Mar 1994 16:43:15 GMT
From: ihnp4.ucsd.edu!agate!library.ucla.edu!europa.eng.gtefsd.com!emory!wa4mei!ke4zv![e-mail]
Subject: CB Mag mount for 10M mobile?
To: [e-mail]

In article <2n77mn$[e-mail]> [e-mail] (Chris Moore) writes:
>What component(s) in an antenna would restrict the power?

Primarily the loading coil. For a short antenna, the currents in the
coil can become huge with only modest amounts of power. They don't use
heavy air core coils that could handle this because it would make the
antenna larger, more expensive, and less attractive to the typical CB
buyer. Look at a Texas Bugcatcher if you want to see the kind of coil
you really need.

The best way to operate 10 meters with a converted CB antenna is to
start with a bumper mount 102 inch whip and shorten it about 10 inches.
No coil, so losses will be only those associated with the poor groundplane
provided by a vehicle. An alternative, not as good, would be something
like a K40 antenna mounted in a drilled hole. These have heavier coils.
Magmounts aren't great at CB or 10 meter frequencies. They couple ground
currents capacitively, and capacitive reactance is greater at lower
frequencies. So they work fine at UHF, but by the time you're down to HF
they aren't very good. This is another of those cases where *drilling the
hole* is the way to go, or at least mechanical bumper mounting.

Icom has a mobile ATU that works with your rig and an 8 foot whip to
cover all HF bands. It works great on 20 thru 10, but the short whip
does begin to limit results on 30, 40, 80, and 160. That's where a
Bugcatcher shines.

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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

Date: 29 Mar 94 16:09:42 GMT
From: sdd.hp.com!hpscit.sc.hp.com!cupnews0.cup.hp.com![e-mail]
Subject: Cushcraft A4 model
To: [e-mail]

Anyone make a successful Elnec/YO model for the cushcraft A4?
Care to share your file?

Jim Hollneback, WA6SDM
[e-mail]

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

Date: 30 Mar 94 03:25:05 GMT
From: dog.ee.lbl.gov!agate!howland.reston.ans.net!noc.near.net!news.delphi.com![e-mail]
Subject: Need advice for HF antenna
To: [e-mail]

Will Perry <[e-mail]> writes:

>There are a lot of hams out there who swear by the G5RV, me included.  Hope
>whatever antenna design you decide on gets you on the air soon.  Good luck.
>
>Will Perry KB0IAR
>[e-mail]

There are a number of us that swear AT the G5RV. On any band except 20M,
a G5RV is losing more than a neglible amount of power in the coax. Throw
away the coax and use ladder-line all the way to the balanced antenna tuner.
Bill Orr, W6SAI, measured an SWR of 6/1 on 15m with a G5RV (CQ Nov '92).
That's more than 4db loss per 100 ft of RG-58 i.e. more than half your power
lost in the coax.

73, Cecil, [e-mail]

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

Date: Tue, 29 Mar 1994 18:17:09 GMT
From: ihnp4.ucsd.edu!dog.ee.lbl.gov!agate!howland.reston.ans.net!europa.eng.gtefsd.com!news.umbc.edu!cs.umd.edu!ra![e-mail]
Subject: Need help resonating 2m dipole
To: [e-mail]

   I recently constructed a 2 meter delta-matched 1/2 wave dipole for my
attic whose basic design follows the one given in William Orr's `Radio
Handbook.'  The delta is terminated into a 4-to-1 coaxial balun which, as
claimed in the book, transforms the 200 ohm impedance of the antenna to
the 50 ohm intrinsic impedance of the coax.  The delta match itself has
the recommended dimensions listed in the book.

   Problem: the antenna doesn't resonate.  The SWR appears to be very
high.  The dipole is a 38.25 inch copper pipe (5/8 inch diameter), which
might be a little long, so I've tried to adjust the impedance by moving
the feed points of the delta-match on the antenna.  (Symmetrically about
the center, of course.)  The next step is to either shorten the antenna,
or change the dimensions of the delta-match.

   One other point: I cut the 1/2 wavelength section of the balun a little
too short.  However, a theoretic analysis of the balun (I'm an electrical
engineer) showes that the effective turns ratio of the balun is not very
sensitive to its length.  Thus, I'm fairly sure the mismatch is not due to
the balun.

   Soooooo... what do I do next?  Cut the antenna?  Change the dimensions
of the delta-match?  Recut the balun?  I need the benfit of someone else's
experience.  I don't want to make any costly mistakes.

-Dave
--
David Drumheller, KA3QBQ          phone: [phone]
Acoustics Division, Code 7140       fax: [phone]
Naval Research Laboratory
Washington, DC 20375-5350   e-mail: [e-mail]

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

Date: 30 Mar 94 03:12:53 GMT
From: dog.ee.lbl.gov!agate!howland.reston.ans.net!noc.near.net!news.delphi.com![e-mail]
Subject: Question about mobile antenna 40/80m
To: [e-mail]

Jack GF Hill <[e-mail]> writes:

>> Also, shouldn't your antenna have capacitive reactance on 10m?
>I would guess it does... though I have no mechanism to accurately
>measure it.

Sorry, I should have said that your antenna has inductive reactance
since it is close to 1/2 wavelength and therefore your coil is hurting,
not helping, your antenna on 10M... unless of course, the coil lengthens
the antenna past the inductive point... does that really work?

73, Cecil, [e-mail]

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

Date: Mon, 28 Mar 1994 17:36:01 GMT
From: elroy.jpl.nasa.gov!ncar!gatech!wa4mei!ke4zv![e-mail]
Subject: TVRO Satellite Dishes on Ham Towers
To: [e-mail]

In article <[e-mail]> [e-mail] (hamilton on BIX) writes:
>
>At the risk of asking a question that's just impossibly stupid, I'll
>do it anyway...
>
>Are amateur radio towers strong and rigid enough to allow a 10 or 12-foot
>satellite dish to be mounted on them?  I'm beginning to give some thought
>to a tower in the 100-foot range but wondering if I might kill two birds
>with one stone, the second being that I'd also like to put up a satellite
>dish but it'd have to be up about 50 or 60 feet to see over the tall
>pines in our area.

Dresser-Ideco used to make one, but it's 9 feet on a face. We've got two
8 foot microwave dishes on ours, one at 300 feet and the other at 980 feet.
You can actually see the 7 inch angle iron cross braces flex in a good wind.

>My sense is that this is asking for a lot in a tower, since the wind load
>of a large disk is enormous and because you can't afford much twisting in
>the wind or you lose the satellite signal.

Your sense is good. The windloading is *enormous*, and the torque that
loading puts on the moment arm of a tower is *incredible*. Our 6 meter
dish, mounted on a 12 foot steel pole about 2 feet in diameter, has a
rated overturning moment of 250,000 ft-lb in a 70 MPH wind. We had to
install a grid of 12 inch I beams in the roof to support it.

If that isn't enough, remember that you typically need a pointing accuracy
of 0.5 degree for a satellite dish. The mounts have to be really rigid to
allow that sort of accuracy.

OTOH, I have seen an eight foot home satellite dish mounted on a 20
foot section of 8 inch well casing filled with cement. It has survived
so far, and been stable enough to keep the satellite in it's wider
beamwidth.

Remember too that most of the satellites will be 22 degrees or more
above the horizon, so you only need a clearance of about 150 feet
from a 60 foot pine in order to see over it from ground level. If
the trees are closer than that, then that's why Homelite makes
chainsaws. :-)

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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

Date: 29 Mar 1994 03:59:06 GMT
From: ihnp4.ucsd.edu!dog.ee.lbl.gov!overload.lbl.gov!agate!library.ucla.edu!europa.eng.gtefsd.com!news.umbc.edu!eff!neoucom.edu!news.ysu.edu!yfn.ysu.edu![e-mail]
Subject: TVRO Satellite Dishes on Ham Towers
To: [e-mail]

>Are amateur radio towers strong and rigid enough to allow a 10 or 12-foot
>satellite dish to be mounted on them?  I'm beginning to give some thought
>to a tower in the 100-foot range but wondering if I might kill two birds
>with one stone, the second being that I'd also like to put up a satellite
>dish but it'd have to be up about 50 or 60 feet to see over the tall
>pines in our area.

Suggest that for more responses to your question in addition to
those received here, you post to rec.video.satellite and/or
email to Gary Bourgois, WB8EOH, at [e-mail].

Gary is one of the resident sat-tv gurus on that newsgroup, and as
you can see, is also a ham.  I'm pretty sure he has mentioned
that he has at least one dish (he must have several in all) on a
tower.

I'm also sure he'd have some advice for you about tall-tower
mounting.  I don't think Gary's is anything as tall as the one
you are contemplating, though.

                                       Hank
                                       N1LTV

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

Date: Mon, 28 Mar 1994 17:12:15 GMT
From: ihnp4.ucsd.edu!library.ucla.edu!csulb.edu!nic-nac.CSU.net!usc!sdd.hp.com!portal.com!portal!combdyn![e-mail]
Subject: TVRO Satellite Dishes on Ham Towers
To: [e-mail]

In article <[e-mail]> [e-mail] (hamilton on BIX) writes:
>
>Are amateur radio towers strong and rigid enough to allow a 10 or 12-foot
>satellite dish to be mounted on them?  I'm beginning to give some thought
>to a tower in the 100-foot range but wondering if I might kill two birds
>with one stone, the second being that I'd also like to put up a satellite
>dish but it'd have to be up about 50 or 60 feet to see over the tall
>pines in our area.
>
>My sense is that this is asking for a lot in a tower, since the wind load
>of a large disk is enormous and because you can't afford much twisting in
>the wind or you lose the satellite signal.
>
I don't know the details.....I didn't get to stop in look...but driving
through the country side....to and from a ham meet....noticed country
homes with what looked like a tower with a dish mounted at the top.  It
wasn't a normal dish.....more like a bunch of horizontal bars arranged
to form a dish.  I guess the use of bars is to reduce the wind load...which
was pretty fierce.....had enough trouble keeping the van on the road that day.

--
 WORK: [e-mail]      | PHONE [phone] | FAX [phone] | VE6LKC
 HOME: [e-mail] |       403 526 6019 |     529 5102 | VE6PAQ
 ----------------------------------------------------------------------------
 Praxis BBS - 529 1610 | CYSNET BBS - 526 4304 | Lunatic Haven BBS - 526 6957
 ----------------------------------------------------------------------------
 disclamer = (working_for && !representing) + (Combustion Dynamics Ltd.);

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

Date: Mon, 28 Mar 1994 22:59:56 GMT
From: agate!howland.reston.ans.net!europa.eng.gtefsd.com!darwin.sura.net!fconvx.ncifcrf.gov![e-mail]
Subject: What length for 2m radials
To: [e-mail]

In article <2n77mk$[e-mail]> [e-mail] (Ronald Viegelahn) writes:
>
>
> If I were to build a 5/8 wave vertical for 2m.
>
> How long should the 4 radials be ?
>
Same as for a 1/4 wave ground plane ie 19.25 inches.

Joe Mack NA3T
[e-mail]

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

End of Ham-Ant Digest V94 #84
******************************