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Author Topic: Rubber Duck Antennas And Gain?  (Read 1629 times)
RADIOHEAD
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Posts: 97




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« on: February 16, 2005, 07:36:33 AM »

In theory, is it true that rubber duck antennas can have gain? Not all rubber duck antennas are unity gain antennas.

I was discussing this with a fellow ham and he mentioned that rubber duck antennas have unity gain only.




 

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K5LXP
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« Reply #1 on: February 16, 2005, 08:57:15 AM »

To get even unity gain on 2M would be a tremendous feat, though on 440 using a flexible whip this is probably true.  Measurements I've done on your basic 6" 2M ducky showed it was about 8dB below a quarter wave.  The groundplane/counterpoise plays a big part in this figure.  Rubber ducks are just a big compromise between efficiency and Q, some models trade one for the other depending on the application.  The same antenna on different radios will perform differently on each one due to the grounding and matching differences, so to give a performance spec on them is difficult to do.  In the commercial 2-way world, antennas are optimized to a specific brand/model for that reason.

Mark K5LXP
Albuquerque, NM
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KE4SKY
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« Reply #2 on: February 16, 2005, 09:33:44 AM »

Flexible helical antennas commonly used on portable transceivers have -5db, “negative gain” compared to a full sized quarter wave whip held at face level. This means that a typical 5-watt portable with helical  antenna has an effective radiated power of about 1-watt.  Placing the HT on your belt produces about -20dB of attenuation, reducing EIRP to approximately 50 milliwatts!    

An expedient often improves performance of your  flexible antenna is a wire counterpoise (24" long for AM air band, 19.5" long for 2-meters, 18" for VHF "high" and marine band; 11” for 222 MHz or 6.5" for the 70 cm band and 6" for GMRS).  A counterpoise prevents transmitted RF from coupling with your body. Your antenna now performs like a center-fed dipole, instead of an "end-fed dummy load!"

Use AWG18-22 gage stranded, insulated wire, crimped and then soldered to alligator style battery clip, which will fit over the outer shield of a the BNC connector on the antenna.  If your HT uses an SMA connector use the proper sized ring terminal to fit over the SMA, enabling you to thread the antenna on over it.  The main lobe of the radiation pattern can be "aimed" by, grasping and pointing the end in the direction where you need a stronger signal.
 
Some after-market and home-made antennas perform much better than the standard helical "rubber duck." Full-sized, flexible 1/4 wave and telescoping half-wave antennas work well.  A full sized quarter-wave whip provides unity gain if used with a counterpoise and held at face level. This represents 5 dB improvement over a typical short helical, because "most" of the effective signal is radiated.

If operating from a vehicle, connect your portable to a magnetic mount mobile antenna to provide a clear RF path outside the vehicle. This overcomes attenuation of -10dB (or more) which results from operating a portable unit enclosed inside a metal vehicle.  

In marginal operating locations a telescoping, half-wave is better, because it provides unity gain without the need for a ground plane. A half-wave antenna can be pulled up into a tree, dangled out a window, attached to a window pane with suction cups, or be used bicycle or motorcycle mobile, or in city driving at moderate speeds on a window clip mount.

A telescoping half-wave increases useable simplex range of a typical 2-meter portable from about a mile with the stock helical antenna to perhaps 3 miles or more, depending upon terrain.  Adding a counterpoise to a unity gain antenna enables a portable to keep in reliable contact within 5 miles of mobile unit parked in a high spot, or an incident command post equipped with an elevated, gain antenna.

Telescoping antennas are more fragile and work best when stationary or in the open, avoiding side impacts or rough handling. Avoid prolonged use of telescoping antennas on window clip mounts over 50 mph, because excessive flexing loosens their internal electrical connections.  

Never collapse a telescoping antenna by whacking it down with the palm of your hand. Gently pull it down with your fingers.  If you note any wobbling or looseness in the sections, replace the antenna.

Flexible antennas are safer when working in close quarters around people and are more durable when walking through dense vegetation for wildfire suppression or search and rescue operations. They "may" be better for dual-band transceivers because telescoping antennas are usually mono-band.   Most dual-band flexible antennas approximate a 1/4 wave on 2 meters and a 5/8 wave on 70 cm, but you should realize that they are usually optimized for one band and may resonate poorly on the other.  How efficient a particular antenna is can be determined only by testing.  Empirical signal strength and readability testing over your usual working terrain and distances are a better indicator of performance than just trying to sweep VSWR with a meter.    

In my experience, a telescoping half-wave, or half-wave, dual-band-mobile antenna used with a magnetic mount, will work well either with or without a ground plane, and offer the best “bang for the buck” for the HT user.  

The spare HT antennas I keep in my go kit are:

Comet CH722A dual-band 39" whip with BNC base which can be used either with a Radio Shack window clip mount, or BNC mag mount; plus telescoping Larsen 2-meter half-wave and a Larsen flexible half-wave UHF whip.

Hope this helps.  
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N3ZKP
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« Reply #3 on: February 16, 2005, 09:44:24 AM »

Unity gain is an oxymoron.

There is either gain, loss or unity.

All rubber duck antennas are loss - typically 5 dB or more, compared to a dipole.

Lon
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AA4PB
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« Reply #4 on: February 16, 2005, 12:24:31 PM »

In theory, is it true that rubber duck antennas can have gain?
---------------------------------------------------
Gain over what? In theory you could place a 1/4 wave long wire inside the rubber coating and have a rubber duck that had about the same performance as a 1/4 wave whip. I guess you could also make one 5/8 wavelength long and get some gain over a 1/4 wave whip. The typical shortened, helical wound rubber duck won't have any gain over a 1/4 whip however.
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K0BG
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« Reply #5 on: February 16, 2005, 12:42:02 PM »

One popular VHF antenna manufacturer openly advertises that their gain figures are based against the average rubber ducky (sure puts new meaning into the term dBd). Their best rated collinear antenna is one with 6.5 dB of advertised gain. If it is indeed rated over an average rubber ducky, then their best (?) antenna has unity gain. Why otherwise well-educated amateurs fall for this exaggerated hype is beyond comprehension. Worrying over the specs of a rubber ducky is of the same ilk.

Alan, KØBG
www.k0bg.com
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WB6BYU
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« Reply #6 on: February 16, 2005, 01:34:16 PM »

Of course a rubber ducky has gain.  You'll never get
anywhere in Marketing if you can't find SOMETHING that
works worse, so you can claim some gain over it.  I'm
sure that you could use a soggy toothpick, dummy load,
earthworm, or wet noodle, among others, as references
against which a rubber duckie would have a positive gain.

Or, try this:

Of course a rubber ducky has gain compared to a dipole.
The numbers are generally negative, of course, but they
still constitute a gain measurement relative to something.

And it would be possible to build a rubber duckie that
had gain compared to a dipole if it were long enough.
I think 3 feet would be long enough on 440 MHz.
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RADIOHEAD
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« Reply #7 on: February 16, 2005, 05:48:37 PM »

Alan,

That's why uneducated hams come to eham.net to ask questions and become educated in the field. But, I see your point and respect your opinion on the matter.

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KB1LKR
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« Reply #8 on: February 16, 2005, 07:12:48 PM »

I think he was expressing amazement that even otherwise experienced hams often seem to accept as gospel truth (and manufacturers continue to publish) gain figures (often w/o dipole or isotropic specified) that are clearly out of whack w/ physics, whether rubber ducks, telecoping whips, or even colinear base & mobile verticals.

Yea, rubber ducks are poor radiators, but convenient and portable -- sometimes they are sufficient for the task, sometimes you need a longer whip/duck, a telescoping antenna, a mag mount on a trash can lid or pizza pan, etc. (though Alan prefers drill the pan and install a good NMO mount :-)

I am also amused by otherwise seemingly knowledgeable hams who get all excited about one FM mobile over another because it has 60W max not 50W max, (a whopping extra 0.8 dB or ~1/8th S unit)!

Keep asking questions, & keep experimenting!
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RADIOHEAD
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Posts: 97




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« Reply #9 on: February 16, 2005, 07:54:31 PM »



When I got low scores in reading comprehension on the SAT's I blamed it on the tests. Now I think I’m going to have to reconsider that.

Thanks for all the responses it was a great read and I learned alot.
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