QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/digests/antenna/940109.txt

Internet Message Format | 1994-11-13 | 9KB

Date: Mon, 18 Apr 94 04:30:06 PDT
From: Ham-Ant Mailing List and Newsgroup <[e-mail]>
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Subject: Ham-Ant Digest V94 #109
To: Ham-Ant


Ham-Ant Digest              Mon, 18 Apr 94       Volume 94 : Issue  109

Today's Topics:
                           bicycle antennas
                           Discone Antenna
Formula for determining optimum distance between elements in a Yagi beam
           Satellite Receive Dishes Combined in Phase Array
                      What to do for RF Ground?

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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: 18 Apr 1994 06:33:55 GMT
From: ihnp4.ucsd.edu!agate!biosci!parc![e-mail]
Subject: bicycle antennas
To: [e-mail]

[e-mail] (John Bate) writes:

>Does anyone have any good ideas about building a bike antenna
>and the mount. I am trying get some info about a decent gain
>2m antenna, probably 1/2 or 5/8 wave. I plan on putting a
>flag on the top to help make my bike visible so height is
>not too much of a problem.

>Anyone have any experience with this?

>john


I recently worked the communications for a local MS walk-a-thon
as "bicycle mobile" using a 2m rig.  I decided that I wanted to
be able to get on and off the bike freely -- I didn't like the
idea of being tethered somehow -- and I liked the idea of the
radio staying with me rather than the bike.  I used an HT clipped
to my belt, a speaker/mic clipped to my shirt, and a homebrew
"helmet mount".

With some help I fabricated a simple bracket that fastens to my
helmet with velcro.  On the top of the bracket is a BNC jack.
Out the back of the bracket is another BNC jack.  This simple,
removeable system has proven to be quite versatile and a very
good performer.

The BNC jack on top allows connection of virtually any BNC-based
antenna.  In my case I have 4 antennas to choose from: a short
rubber-duck about 4", a longer rubber-duck about 8", a 1/4 whip,
and a 5/8 wave collapsible.  I can ride with any of the first 3
without any problems.  The 5/8 is to be used while stopped,
if needed.

The second BNC jack on the bracket allows me to pick and choose
any length or type of coax that I want to connect to the HT,
depending on the situation.

The velcro allows me to remove the entire bracket in seconds in
order to return my helmet to normall use.  I had some doubts
about the velcro being strong enough to hold everything stable
while in use but I encountered no problem.  Things were remarkably
stable.

Since the antenna is up as high as possible and clear of any
obstructions it works quite well, whether you're on the bike or
off.  Also, since all of the antennas are actually HT antennas
you are free to use them either on the helmet mount or directly
on the HT, whichever best fits the current situation.

The BNC jack and/or the velcro enables you to remove the antenna
in seconds (the bracket itself is very low-profile and virtually
disappears without an antenna connected).  This can be quite
usefull when making a pit stop of one sort or another.  The only
drawback that I encountered were the various giggles and remarks
that I would occasionally get from people who had never seen an
antenna sticking out of someones head before.  Oh well.

All-in-all I was quite happy with the entire setup.  My helmet
has the little velcro patches stuck to it -- all ready to go
when I need it.  The only thing that might change is to try a
headset rather than the speaker/mic; maybe next time...

Wes Irish, WA2CRQ
[e-mail]

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

Date: Sun, 17 Apr 94 23:43:48 GMT
From: mnemosyne.cs.du.edu!nyx10![e-mail]
Subject: Discone Antenna
To: [e-mail]

For some really good information on discones, try to find the old
CQ Anthology, published back in the 60s.
They reprinted a whole series of articles on discones.
I use a Radio Shack discone, and I am told Icom makes a better one.
It does seem to have a nice low angle of radiation on two meters.

--
 [e-mail]   Robert Davis  Salina, KS
                        Amateur Radio K0FPC

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

Date: Sun, 17 Apr 1994 22:11:07
From: ihnp4.ucsd.edu!swrinde!gatech!newsxfer.itd.umich.edu!zip.eecs.umich.edu!panix!ddsw1!elink.pr.mcs.net![e-mail]
Subject: Formula for determining optimum distance between elements in a Yagi beam
To: [e-mail]

Is there a formula for determining the optimum distances between the driven
element and the parasitic elements in a yagi beam?

I know the driven element should be a 1/2 wavelength and the reflector 5%
longer and the director 5% shorter in a 3 element beam.  How do I determine
the distance between the driven element and the parasitic elements?  What
about if the yagi has more than 3 elements, say 7?

Any help would be most appreciated.

Thanks,
George


--

   Email: [e-mail]  |    One week down in the waiting
Ham call: <pending>        |    period for my ticket.

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

Date: 18 Apr 1994 00:28:21 GMT
From: ihnp4.ucsd.edu!swrinde!cs.utexas.edu!uwm.edu!convex.csd.uwm.edu![e-mail]
Subject: Satellite Receive Dishes Combined in Phase Array
To: [e-mail]

Is anyone aware of successful methods for combining two or more satellite receive dishes
in phase array as a means of achieving receive gain comparable to a single
larger dish?  Is there a Usenet Group concerned with the engineering aspects
of satellite broadcasting and receive technology?  Thanks

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

Date: 14 Apr 94 05:47:35 GMT
From: agate!howland.reston.ans.net!gatech!newsxfer.itd.umich.edu!nntp.cs.ubc.ca!cyber2.cyberstore.ca!nwnexus!ole!rwing!eskimo![e-mail]
Subject: What to do for RF Ground?
To: [e-mail]

In article <2oic53$[e-mail]>,
Christopher D. Sorensen <[e-mail]> wrote:
>I have my station on the second floor of my house and unfortunatly
there is
>no water pipes or anything of the sort for a decent RF ground.
>
>I have been told continuously that the run to the earth ground must be
short
>for it to be effective.  Would it be ok to have a pipe driven in the
ground
>outside just as though I had a groundfloor station and just run the
ground wire
>down outside to the ground rod?  Or would  this just be a waste of
time?
>
>I don't get any bites, but I would sure like to have a good rf ground
system.
>especially when I eventually run more power.
>
>Any info appreciated.
>
>Chris -=- N8PBI
>


Contrary to ancient and widespread opinion, Chris, you don't NEED an RF
ground on your rig at all, with one exception:  when you are
intentionally using the ground as part of the antenna itself.  An
example would be a ground mounted vertical with no radials, where the
coax braid would be connected to a rod driven into the ground.  (A lousy
antenna, by the way, due to guess what - ground losses!).

For antennas such as dipoles, loops, beams, etc, forget the RF ground.
You want your hard-earned RF up in the air where it belongs, and as far
away from the earth as possible.

Now this is not to say your rig shouldn't be grounded - it should for
safety - but the "third pin" on the ac plug will do the job just fine.
This is not an RF ground though, just a 60 Hz one.

You say you're not getting any RF "bites" off your rig, which is good.
If you ever do though, please don't just try to "ground" it.  Anytime
one has RF in the shack, one has a problem in the antenna system.
Almost always, the problem is due to unbalance in the antenna, or less
commonly, in the feedline.  Fix the unbalance if at all possible, or if
you really can't, then at least put a balun or isolating type tuner in
the feedline as appropriate.

Grounding for lightning protection is a whole 'nother subject, which I
will leave to the experts.  Us Pacific Northwesters don't hardly know
what it is :->

RF energy is expensive to generate - don't waste it by grounding it!


73 es gl


Bill, W7LZP



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End of Ham-Ant Digest V94 #109
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