Subject: [NEXTEL-1] Antenna theory

From: [email-masked]
Date: Wed, 02 Jul 2003 23:04:38 -0400

-------- Original Message --------
From: Mark Krotz <[email-masked]>
Subject: Antenna theory

Can anyone point me to a reference, or comment on the following:

In the example of an exposed dipole design antenna (ie, DB222, DB224), 
how does the spacing between bays affect the gain of the antenna? 
Probably best to think in terms of a 2 bay antenna.

I know that there is an optimum max gain spacing, and increasing or 
decreasing the spacing from that lowers the gain.. but interested in a 
deeper look at it.

Also, interested in the affect on gain, tilt, etc if the bays are fed 
out of phase, and same question for "corporate feed" (all bays receive 
the same power) vs. series fed (power varies between bays)?

M

------------------------ Yahoo! Groups Sponsor ---------------------~-->
Get A Free Psychic Reading! Your Online Answer To Life's Important Questions.
http://us.click.yahoo.com/Lj3uPC/Me7FAA/ySSFAA/61FxlB/TM
---------------------------------------------------------------------~->

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:29:35 -0400

------- Original Message --------
From: Jack Daniel <[email-masked]>
Subject: Re: Antenna theory

I thought broadcast used cross-polorization or dual-polaroization. 

Generally, doesn't using circular polarization for multipath work best
if BOTH antennas are circular polarization? 

That way a reflected signal has reverse rotation when hitting the
receiving antenna.

But I may be wrong...
Jack Daniel
=================================

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:26:44 -0400

------- Original Message --------
   From: "Maples, Dave" <[email-masked]>
Subject: RE: Antenna theory

 I believe that you change downtilt by changing (slightly) the length of some of
the phasing harness elements. You may find some material on Cebik's web site
(www.cebik.com) that applies here, or you might try the antennex web site.I'd love
to spend more time learning about antennas... Dave Maples

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:24:19 -0400

-------- Original Message --------
From: Mark Krotz <[email-masked]>
Subject: Re: Antenna theory

> 
> The dipole spacing is wavelength related. I'll try to look it up in my
> VERY old files.

DB says max gain 3.3 dBd spacing is 0.9 wavelength.  Gain drops to 3 dBd 
at .75 and 1.25 wavelength, and down to under 2 dBd at 1/2 wavelength, 
and down to about .6 dBd at 1/4 wavelength.

> 
> I do know if you have the spacing correct but invert one of the dipoles
> it basically cancels another dipole. 
> 

Interesting.. I've seen exposed dipole type broadcast antennas 
intentionally fed out of phase.. in particular, one 4 bay that has the 
two center bays fed in phase with each other, and the top and bottom 
bays are in phase with each other, but out of phase with the center 
bays.  But then since this antenna appeared to be series fed, I'm not 
sure what the feedpoint phase differences are.  I was told at the time 
that they were fed out of phase to affect the vertical aperture.

DB says feeding bays out of phase would affect tilt, but didn't mention 
anything about how it would affect gain.

> 
> I'll post the spacing formula when I find it.
> 

I'd like to see that.  I guess another factor would be the wavelength of 
the dipole.

Thanks,
Mark

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:28:01 -0400

-------- Original Message --------
From: Jack Daniel <[email-masked]>
Subject: Re: Antenna theory

Dave,

I meant the gain cancellation in a standard 212 occurs when you invert a
dipole when the harness wasn't made to do that. In the broadcast antenna
the harness WAS designed to have alternating elements inverted.

I should have paid more attention to Tom McMullen (the father of the 212
and all it's offspring) when he explained it to me. The original version
was a CE-212 model. "CE" representing Communications Engineering Company
(CECO)in Dallas. The Decibel brand started when CECO decided to become
Communications Industries Inc., separated their many operations into
stand alone subsidiaries; CECO (GE service and sales), Decibel (antennas
and filters), Com-Supply (GE parts distributor), Telemote (remoite
control and equipment mfg), Mobilefone (common carriers)  and
CECO-Automation (SCADA and signalling systems - later changed to Secode)

Now that I've drifted completely off subject, I'll shut up. ha ha.

Jack Daniel

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:30:22 -0400

-------- Original Message --------
   From: "Maples, Dave" <[email-masked]>
Subject: RE: Antenna theory

 Jack: The idea is that if a linearly-polarized receiving antenna is looking at a
circularly-polarized transmit antenna, the probability of a complete multipath
null is very low because one polarization will probably be present when the other
one fades.  At least that's the way I understand it.  Looking up in my antenna
book, the circular polarization is done to ensure that if the polarization at the
receiver is random (as it would be after re-entry from a satellite in space) that
signal can be recovered. For what it's worth...Dave Maples

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:28:50 -0400

-------- Original Message --------
From: Mark Krotz <[email-masked]>
Subject: Re: Antenna theory

> 
>> What about possibly trying to create a circularly-polarized waveform 
>> to deal with multipath?
> 

> I think you may have fewer cobwebs than some of us, Dave. Now that you 
> mention it, I believe that is precisely the intent in broadcast antennae;
> 

Yes.. but I'm not concerned about multipath in my reasons for being 
curious here.

The broadcast antenna I mentioned was vertical polarization only, which 
I found odd.  I've seen plenty of stations horizontal only, and plenty 
circular polarized, but not many vertical only.

In fact, unless I'm mistaken (and I could be), I think the rules only 
allow non-commercial stations to run vertical only.

BTW, I don't recall seeing too many circular polarized antennas in LMR 
(or even paging) installs, even were multipath is an issue.

Maybe the "half ERP" thing scares people?  ie, if you have 1kw and a 3dB 
gain antenna, you have 2kw ERP.  But if you're circular polarized, you 
have 1kw ERP vertical, and 1kw ERP horizontal (more or less).

M

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:25:05 -0400

------- Original Message --------
From: Jack Daniel <[email-masked]>
Subject: Re: Antenna theory

Mark,

I think the physical difference between the broadcast array arrangement
maybe occurring within the coax harness. If you added a 1/2 wave coax
section between the DB dipoles you could probably invert every other
dipole.

There is another factor called 'beam sweeping'. The beamwidth and the
beam launch angle (i.e. amount of 'tilt') changes over a wide bandwidth
and the effect get worse as gain is increased. In a multiple dipole
array, there is also inter-action between the dipoles. In a wideband,
high gain broadcast antenna, the alternating dipole fields might (I;m
noit positive} cause a stabilization of the overall launch angle (one
tilts 'up' the other tilts 'down') but there would be soem change in the
vertical beam width.

Your primary lobe could move quite a lot causing coverage to change for
each channel. A SMR with widely spaced channels could chase their tails
a long time trying to solve intermittent drop-outs at the outer edge of
coverage.

This effect is most noticable in bottom feed colinears because the
dipoles are all fed from the bottom. DB has tried to compensate for this
in some models by center feeding two colinears, resulting in the lower
half being fed fom the top. The beamwidth still changes over the
frequency bandwidth.

These effects can be minimized with an external harness or internally
the way the new TX RX Systems antennas are constructed. It's called 
'corporate feed'.

The 0.9 may relate to cutting the harness length as that approximates
the propagation rate of a lot of coax cables. I'm guessing the dipole
spacing is closer to a factor of 1.0 since it's air dialectric.

Tearing through a lot of old mental cobwebs on this end. ha ha

Jack Daniel

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:22:29 -0400

------- Original Message --------
From: Jack Daniel <[email-masked]>
Subject: Re: Antenna theory

Mark,

When 212-2's first it was a common symptom to have about the equivalent
to a dummy load at the top of the tower when that mistake was made. The
first versions did not have the red orientation tape, only a drain hole
on the lower element.

I have been told you can get a small down-tilt (or up-tilt) by changing
the dipole separations, but never personally verified that.

Jack Daniel
=======================================

---

From: [email-masked]
Date: Wed, 02 Jul 2003 23:26:01 -0400

-------- Original Message --------
From: Mark Krotz <[email-masked]>
Subject: Re: Antenna theory

> section between the DB dipoles you could probably invert every other

Well, this would get back into the point I mentioned earlier of 
"corporate feed" vs. series feed.

Corporate feed would use power divider/coax network to deliver the same 
power to all bays.

Series feed would result in different power levels being delivered to 
each bay of the series leg (as happened on a series, bottom fed 
collinear), and there would be some phase differences, if not somehow 
compensated.

In either case, I guess the bays could be fed out of phase from each other.

> There is another factor called 'beam sweeping'.

Yes, familiar with this.  This effect is aggravated by moving off of the 
resonant frequency of the antenna.. whether it be that the carrier 
frequency changes, or the carrier with intelligence is that wide.  ie, 
if you had an antenna exhibiting this effect, and the transmitted signal 
was a conventional NTSC TV signal, you would have a different downtilt 
on the aural carrier than on the visual carrier, because of their 
difference in frequency.  Or as you mention in a SMR trunk system, where 
the frequencies combined into a collinear type antenna were spread out.

 > In a multiple dipole
> array, there is also inter-action between the dipoles. 

True.. according to DB, this effect becomes more noticeable when you 
start getting closer than 1/4 wavelength spacing.

 > In a wideband,

Rather than cancel each other out, as in the DB212 example?

> spacing is closer to a factor of 1.0 since it's air dialectric.

I assumed it would be a velocity factor thing?  In the info from DB, it 
is mentioned the harness remains the same with the different spacing.

> Tearing through a lot of old mental cobwebs on this end. ha ha
I hear ya.

Mark