Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: What makes a dual band antenna "dual"  (Read 1512 times)
KE5HJO
Member

Posts: 207




Ignore
« on: August 10, 2006, 03:57:32 PM »

I was reading an article in this months QST magazine (Page 50) on how to build a simple, dual band sleeve antenna.  Very interesting article and a project I might try myself.  However, I got to wondering what made this particular antenna a dual band antenna.  Then I thought, in general, what makes an antenna dual in a first place.

I know that a single band antenna resonates at a certain frequency and I know you can control an antenna's frequency by changing its length.  So, what makes a dual band antenna work when you only have one piece or "length" of wire to work with?

I can take the technical stuff so get as deep as necessary.

Thanks,

Mike
Logged
KA4P
Member

Posts: 94


WWW

Ignore
« Reply #1 on: August 10, 2006, 05:03:57 PM »

Ummmmmmm cause it will resonate and hopefully radiate on two bands?
Logged
WB6BYU
Member

Posts: 13482




Ignore
« Reply #2 on: August 10, 2006, 05:12:59 PM »

A vertical whip will be resonant on multiple frequencies
at the same time - approximately at any multiple of 1/4
wavelength.  ("Resonant simply means that the feedpoint
impedance is purely resistive - with no reactance.)  On
even multiples of a quarter wavelength the impedance is
very high, so it isn't a good match to coax, but on odd
multiples of 1/4 wave (1/4, 3/4, etc.) the impedance is
relatively low.  So a 1/4 wave whip for 2m will also
resonate as a 3/4 wave whip for 440 MHz, making it a dual
band antenna.  One problem, though, is that the radiation
pattern for the 3/4 wave whip isn't as tidy as one might
like, with more radiation going off at the higher angles
than more usefully at the horizon.  So often such antennas
are modified in such a way as to improve the pattern on
440 without significantly affecting it on 2m.

In general, "dual-band" simply means that the antenna will
work on two bands, and this can be accomplished in many
different ways, i.e. with traps, parallel elements cut
for different frequencies, multiple resonances, etc.
Logged
RobertKoernerExAE7G
Member

Posts: 1435




Ignore
« Reply #3 on: August 10, 2006, 06:04:55 PM »

http://www.athensarc.org/sleevedipole.htm
Logged
RobertKoernerExAE7G
Member

Posts: 1435




Ignore
« Reply #4 on: August 10, 2006, 06:21:41 PM »

"It is very broadband, being usable from about 142 to 152, and all of the 440 band as a 3/2 dipole."

A 40 meter dipole also works for 15 meters.

If I remember correctly, W4RNL's site has good info that will give you more background on operating one antenna on multiple bands.

Also note, that many of our HF bands are harmonically related: 3.5, 7, 14, 21, 28.  At one time the second harmonic of 3.5 xral oscillator was regularly used for 7Mcs.

You might have fun thumbing through any old handbook from the 40s or 50s.

73
Bob
Logged
KE5HJO
Member

Posts: 207




Ignore
« Reply #5 on: August 10, 2006, 06:50:46 PM »

Okay.  That makes sense.  But I sometimes read where a certain antenna uses a loading coil, choke, etc. that helps make that certain dual or multi band.  Why and when are these necessary?

Thanks,

Mike
Logged
K0BG
Member

Posts: 9886


WWW

Ignore
« Reply #6 on: August 10, 2006, 07:50:35 PM »

The most popular dual band amateur antenna is the Larsen NMO 2/70B. It is a loaded 1/2 wave on 2, and two 1/4 waves on 440. In the center is an air-wound phasing coil. It is not only rugged, it is almost stealthy in appearance.

While the advertised gain doesn't "match" that of some other, overrated, import antennas, the performance speaks for itself.

Alan, KØBG
www.k0bg.com
Logged

KE5HJO
Member

Posts: 207




Ignore
« Reply #7 on: August 10, 2006, 08:47:57 PM »

Okay - here is a perfect example of what I am talking about.  This particular antenna has a center, air-wound phasing coil.  Why?  Is it because it is 1/2 wave on 2m and, according to previous posts, resistance is high on 1/2 wave compared to 1/4 wave?  I guess my question is - what does this phasing coil do and how does one know when and where to use one?

Mike
Logged
KA5N
Member

Posts: 4380




Ignore
« Reply #8 on: August 11, 2006, 03:04:45 AM »

The QST article doesn't have much in the way of explaination of the theory of operation.  Notice, however that the antenna consists of two sections.  One is 15+ inches and the other is 21+ inches.  The lower 15 inch segment resonates on 70 cm and the upper 21 inch segment is resonant on 2 meters.  Only one segment responds to your signal depending on which band you are using.
Since you are trained as an engineer might I suggest that you get some antenna books and indulge yourself as deep into antenna theory as you desire.  Look at Cebik's offerings they will keep you busy for a few weeks.  The excellent engineering text "Antennas" by John Kraus will provide all the heavy math you can take.  You can probably get a used copy at a college book store since I believe it is out of print.  My copy cost $21 used.
GL Allen
Logged
KE5HJO
Member

Posts: 207




Ignore
« Reply #9 on: August 11, 2006, 05:21:15 AM »

I actually borrowed that book but I had to return it.  It is very indepth and I was reading it page for page as time permitted.  Great book!

Okay - I understand that one length is resonant on 440 and the other on 144.  I do not understand why a 1/4 wave 2m/70cm has the same duality using just one length of wire and some extra inductance and requires a ground plane to be effective.  I guess that part I need to figure out on my own.

I tried modeling this antenna in Eznec to get an idea of what was going on.  The performance was okay until I introduced a ground plane.  Of course, my skills with this program are minimal at best.

Thanks for all the help.  I need to dive into antenna theory - off to Amazon.com!

Mike
Logged
AA4PB
Member

Posts: 12985




Ignore
« Reply #10 on: August 11, 2006, 07:18:27 AM »

I do not understand why a 1/4 wave 2m/70cm has the same duality using just one length of wire
--------------------------------------------------
Because it's only a 1/4 on 2M. It's 3/4 wave on 70cm. Both are resonant lengths although the feed impedance will be different.

All antennas require a ground plane or counterpoise of some sort unless they are fed in the center (i.e. balanced).
Logged
KE5HJO
Member

Posts: 207




Ignore
« Reply #11 on: August 11, 2006, 07:54:16 AM »

Ah...now I get it. Thanks for clearing that up.

Now, I've just modeled a simple 1/4 wave antenna in EZNEC.

Length = 20.21
Freq = 146 MHZ
Ground = Real/MININEC
Source at 0,0,0
End 1 at 0,0,0

The gain is suprisingly -0.19 dBi.  This is the same type antenna I use on my mobile rig and I know I have more gain than -0.19 dBi.  I thought 4 to 5 dbi is common for this setup.  Am I modeling this incorectly in EZNEC?

Mike
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #12 on: August 11, 2006, 08:22:14 AM »

Two things..

1)you know what your absolute gain in dBi is?   How'd you measure it?  You won't very often notice a 4dB difference on FM...

2)Your mobile antenna is not on the ground.  It's probably up about 0.75wl over a fairly good conducting groundplane .. a better model might be over 30 1/4wl radials , up the same height as your car roof above ground, and use Real/High Accuracy ground.  

Slightly less than 0dBi is correct for a ground mounted vertical over real ground with a good on-ground radial system underneath.

The car provides a good ground return path for currents near the antenna, but doesn't do everything to set up the far-field pattern of the antenna; the fact that the antenna is elevated above ground contributes some gain.

73,
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
Member

Posts: 13482




Ignore
« Reply #13 on: August 11, 2006, 12:17:58 PM »

Are you feeding your antenna against ground?  Often it is
easier to start with a dipole in free space first.  Try
a 40" wire that is fed in the center.  It should show
around 72 ohms at resonance (depending on the wire diameter.)
Then change the frequency to 445 and see what it looks
like.  Probably not a perfect match to 50 ohm coax, but
reasonably close.  Then look at the radiation patterns
and compare the amount of radiation perpendicular to the
wire.

You can do the same thing using a 20" wire over a ground
plane (or real ground) with similar results.


For the Larsen dual-band antenna, the center loading coil
is self-resonant on 440.  This allows it to act effectively
as an insulator between the two halves of the antenna,
and it also causes a 180 degree phase shift from one
end of the coil to the other.  This phase shift is
important, because without it the radiation from the
two adjacent half-wave sections would be out of phase,
which is the cause of the lobes in the original dipole
pattern on 440 MHz.  So on 440 the antenna consists of
two half waves in phase with the phasing coil between
them.  On 2m, each half of the antenna is about 1/4
wave long.  The loading coil is at the point of maximum
current/radiation, so it isn't as efficient as a straight
whip might be.  The presence of the coil at that point
causes some electrical lengthening of the antenna so it
appears at the feedpoint like a 5/8 wave whip rather
than 1/2 wave, and the matching network at the base of
the antenna is designed to match the antenna to coax on
both bands.

Often you will see a shorter version of this, basically
a quarter wave whip with a small open coil about 1/3 of
the way up the whip.  On 440 this is a 1/4 wave radiator
in the bottom, with a phasing coil and a half wave
top radiator (both in phase.)  On 2m this is close to
quarter wave resonance, so it is suitable for direct
coax feed on both bands.

Note in both cases that the coil is designed to be self-
resonant on 440.  This is an important part of the
design, and isn't obvious if you just look at the turns
of wire as a coil.  They are carefully designed to have
the right self-capacitance to make the antenna work.

Here is a good analysis that might help you understand it:

http://www.cebik.com/vhf/cc.html
Logged
RobertKoernerExAE7G
Member

Posts: 1435




Ignore
« Reply #14 on: August 11, 2006, 12:59:09 PM »

Ooooops.  Now you've done it Mike.  You've started down the "How do antennas work?" road.

Before you hit Amazon, try eBay.  I love ON4UNs book, "Low Band DXing", third edition or fourth edition--tons of antenna info.  Any ARRL HandBook has great info--cheap on eBay.

Plus, W4RNL has tons of info on line.
73
Bob

PS: whatever you do, do not decide to build a tube power supply for a one tube transmitter.  Way too much fun for one person to handle.
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!