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Oct 24, 2007, 02:16 PM
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dalbert02's Avatar
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Smiley Rubber ducky, tuned to what frequency?


A long time ago, I purchased a Smiley Rubber Ducky. At that time I was on 72Mhz. Then I went to 50Mhz. I called Smiley and asked if I needed to buy a new 50Mhz antenna. They said, "No, they are the same antenna." Curious to know how an antenna could work equally well on both 50Mhz and 72Mhz, I put the antenna on my FMJ-269 antenna analyzer. The antenna had the lowest SWR at 68.3Mhz. At any 72Mhz or 50Mhz frequency the SWR was nearly infinite. Since I had the analyzer, I decided to add some length until I had it tuned to my Tx frequency of 50.860. Perhaps if you are getting poor performance with your rubber ducky, it is because it was improperly tuned. I imagine they tune for a middle frequency as they do not know what frequency you will be on. In my opinion, the antenna should be purchased like a crystal - good for only one channel. Something important to remember is that a good SWR (correction-meant load) does not imply a perfect antenna. If the load has mostly reactive impeadance (capacitive reactance or inductive), even if it is a perfect 50 ohms, the antenna will perform poorly as reactance does not absorb power. Even though my impeadance is slightly higher then 50 ohms, it is all resistive, which readily accepts power.

-dave

edit corrected for my mistake
Last edited by dalbert02; Oct 26, 2007 at 08:30 AM.
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Oct 24, 2007, 02:35 PM
Inciting Riots
village_idiot's Avatar
I was going to buy one of those.... Glad I didn't.
Oct 24, 2007, 04:07 PM
Registered User
Ping's Avatar
I don't really agree that you should buy an antenna for each freq. That would be a little extreme.

For sure though that antenna should be tuned to either the 72mhz band or the 50mhz band. Going halfway isn't acceptable in this case.

Incidentally I have the same antenna analyzer, You should of just wound your own base loaded antenna if you already had the analyzer. I've made a few base loaded RX antennas with mine also.
Oct 24, 2007, 04:12 PM
Registered User
Using a Rubber Ducky will decrease your maximum range. Fortunately, even with that loss in range, most present day transmitters work okay. Check out rc-cam's site for some good info on what effects antenna length has on both transmitters and receivers. In nearly all cases, a loss occurs, usually cutting effective range in half or less.
Oct 24, 2007, 04:26 PM
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dalbert02's Avatar
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I had the antenna before I bought the analyzer, but if I knew then what I know now, well, I guess I would just make my own antenna.
-dave
Oct 24, 2007, 04:31 PM
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ebill3's Avatar
Well, no surprise that the the duckies are the same for 50 mHz and 72 mHz, as the as supplied whips are also the same size. And, I believe the same whip is used for European frequencies as well.

The modified ducky now looks better to the analyzer, but does it look better to the transmitter?

Bill
Last edited by ebill3; Oct 24, 2007 at 04:56 PM.
Oct 24, 2007, 07:33 PM
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dalbert02's Avatar
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Well, if the transmitter has an output impeadance of 50 ohms, then yes. If not, then no. I guess an experiment is in order to see if my range has increased.
-dave
Oct 25, 2007, 01:05 AM
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ebill3's Avatar
Quote:
Originally Posted by dalbert02
Well, if the transmitter has an output impeadance of 50 ohms, then yes. If not, then no.
Of course, that was the crux of my question. I do not know, but it seems logical to assume that if the same length whip antenna can serve several frequencies, then the output stage is designed for an impedence to match that antenna at that frequency. It may or may not be 50Ω.

Also, there is the other half of the antenna to consider. The internal metal and wires of the transmitter, and perhaps the human holding it?

All that being said, antennas are sort of PFM - I hope your mod does increase range.

Bill
Last edited by ebill3; Oct 25, 2007 at 01:24 AM.
Oct 25, 2007, 08:45 AM
Registered User
Check out www.rc-cam.com's site for some good info on antenna pruning. In most cases you will find that you will always have less range with a rubber duck regardless of brand than you will with the conventional whip antenna. Usually you will lose 3 to 6 db or 1/2 to 3/4 of your power density.
Oct 25, 2007, 12:45 PM
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dakir's Avatar
Hello dalbert02.
I for a long time itself do short TX antenna. Recently has decided to tell about the manufacturing techniques. If it is interesting to you to look here.
https://www.rcgroups.com/forums/show...=663864&page=2
Last edited by dakir; Oct 25, 2007 at 01:42 PM.
Oct 25, 2007, 08:46 PM
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dalbert02's Avatar
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Quote:
Originally Posted by ebill3
Also, there is the other half of the antenna to consider. The internal metal and wires of the transmitter, and perhaps the human holding it?
Just FYI, I did my tuning while standing and holding the base of the antenna and the analyzer ground plug. If I placed the antenna and analyzer on a table then it would no longer be tuned and the SWR meter would be all over the place.

I heard or read somewhere that all FCC approved rf transmitting devices are tuned at 50 Ohms. Maybe someone could prove or disprove that? It seems logical as almost all commercially available coaxial cables are 50 ohm except tv & video at 75 ohm.
-dave
Oct 25, 2007, 09:21 PM
Registered User
I'm a little late getting in on this thread. A couple of comments--

Quote:
Something important to remember is that a good SWR does not imply a perfect antenna. If the load has mostly reactive impeadance (capacitive reactance or inductive), even if it is a perfect 50 ohms, the antenna will perform poorly as reactance does not absorb power. Even though my impeadance is slightly higher then 50 ohms,
A reactive load of 50 ohms does not give a SWR of 1:1. Theoretically it gives an infinite SWR. On my MFJ 269 it gives 20:1 (which is due to the limited range of the measurement circuits).

Quote:
Also, there is the other half of the antenna to consider. The internal metal and wires of the transmitter, and perhaps the human holding it?
I posted some work I did on the Rubber Duck and Transverse Dipole antennas on my website. The item on the Transverse Dipole addresses the issue of the person holding the transmitter. For those using 35MHz the loss is huge (15-20 db); at 50 MHz is around 10 db. At 72 MHz it is small.

http://mysite.verizon.net/res7yvp2/w4dh22/

Quote:
Well, if the transmitter has an output impeadance of 50 ohms,
The usual RC transmitter is designed to match a capacitive reactance and resistance. I checked several Futaba transmitters by placing a series capacitor and resistor from the pad for the pigtail that connects to the based of the anntenna and ground and found the values that gave the most power into the resistor. The capacitor was about 17pf and the resitance was about 33 ohms. For 50 MHz this is roughly the impedance of a 39" whip.

Quote:
I heard or read somewhere that all FCC approved rf transmitting devices are tuned at 50 Ohms.
I have never heard that, and it makes no sense for the FCC to control that. The reason we see 50 ohms is that theoretically it can be shown the minimum loss occurs when a coax is about 72 ohms, but with an air dielectric. Add a dielectric that has a higher "e" and that brings it down, so for the conventional coax, 50 ohms around the optimum. We don't see 10 ohms, nor several hundred in coax, as the loss would become high. To get the transmitter power to the antenna by using a coax the transmitter would logically be designed to output into 50 ohms. When the antenna connects directly to the transmitter, such as what we have with the typical rc transmitter, then it is a different problem.
Oct 26, 2007, 06:43 AM
My member is not
Zlatko's Avatar
Hi,

A while back I stumbled accross info on "The Hairpin Monopole Antenna"
http://www.antennex.com/preview/monopole.htm

Any comments?

Cheers
Oct 26, 2007, 06:56 AM
Alt+24=↑ Alt+25=↓ Use num keys
Miami Mike's Avatar
I'm also suspicious about my two Smiley antennas. When I bought them, the company gave me every indication that I was getting antennas specifically made for the 50 Mhz band, which is what I wanted and what I ordered. But when I got them there were no indications on the antennas, in the instructions, or on the packaging that they were specially meant for 50 Mhz. For several years I've been using both antennas on my two JR XP8103 radios, which I can change from channel 05 (50 Mhz) to channel 36 (72 Mhz) by swapping modules. I've had no range problems on either band. I think the 50 Mhz and 72 Mhz antennas are the same, but I'm not aware that Smiley is currently owning up to it.

Incidentally, I prefer rubber duck antennas because they theoretically give greater range that the stock telescoping antenna when your radio is held in the most natural and common position relative to the direction to your model. The common misstatement that they shorten your range is a myth, unless you habitually hold your radio sideways or in front of your face. Telescoping antennas typically transmit most of their signal sideways.

And one other thing - There's no such thing as SWR if there's no feedline for the standing waves to stand on.
Oct 26, 2007, 06:56 AM
Registered User
dakir's Avatar
Quote:
Originally Posted by dalbert02
Just FYI, I did my tuning while standing and holding the base of the antenna and the analyzer ground plug. If I placed the antenna and analyzer on a table then it would no longer be tuned and the SWR meter would be all over the place.

I heard or read somewhere that all FCC approved rf transmitting devices are tuned at 50 Ohms. Maybe someone could prove or disprove that? It seems logical as almost all commercially available coaxial cables are 50 ohm except tv & video at 75 ohm.
-dave

Hello dalbert02!
I used for the aerials cables 50ом and 75ом differences big are not present.
As the core works in other mode.
Exact data by means of the analyzer in house conditions to receive very difficultly.
It is necessary to consider many errors. For such measurements there are special laboratories. Therefore I in the aerial have reduced adjustment to a minimum for construction of the antenna by any person and reception of good result.


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