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Need expert advice on thin antena vs reception performance

Old 09-22-2010, 05:24 PM
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Default Need expert advice on thin antena vs reception performance

Antena above water always tend to ruin the look of rc subs[].
I am planning to use the thinnest antena available, hopefully it is so thin that it is invisible from a few feet. Anybody has experience how thin is TOO thin so it start to affect the radio reception
Old 09-22-2010, 06:05 PM
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Default RE: Need expert advice on thin antena vs reception performance

First off, diameter doesn't really count for anything.

Second, why even bother with the antenna sticking up? As long as you're in fresh water, just lay it out along the side of the hull. In close to 40 years of building these things I have NEVER had an antenna that was above the surface even when the boat itself was surfaced.

Skip Asay
Old 09-22-2010, 06:37 PM
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Default RE: Need expert advice on thin antena vs reception performance

Difference is that my transmitter with a only 4 inch antena is powered by the one dollar tiny square 9V battery & the RC board is powered by 3 tiny AAA ( 3 for 1 dollar) batteries. I need every edge under the sun to maximize the reception without ruining the good looks of the' true to scale ' models .
Old 09-22-2010, 07:38 PM
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Default RE: Need expert advice on thin antena vs reception performance

Yes I would make it thin and verticle in this case since the TX is not strong and once submerged reception will get worse,how thin i tried once with a single strand of brass  wire about 2 hairs in diamiter and about 2.5 ft long in a plastic tube for support  it maid a huge difference.

Dave
Old 09-22-2010, 08:18 PM
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Default RE: Need expert advice on thin antena vs reception performance

Best bet is to do whatever is possible/necessary to boost the output of the transmitter. Increasing the ability to receive won't do a heck of a lot if there isn't much there to receive. Any attempt to change the length of either receiver or transmitter antenna MUST be done properly. Don't just add a bunch to it. What this means is that these antennas are (should be) tuned to the wavelength used so, if the receiver antenna is 10" long then you would want to increase that in multiples of 10". I believe the same holds true for the transmitter, as well. BUT, that isn't exactly my area so keep looking until you find an RF guy.

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Old 09-23-2010, 07:00 AM
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Default RE: Need expert advice on thin antena vs reception performance

subicman has done lots of research on this on his own and for the navy. He could tell you just about anything you want to know. you should send him a PM.
Old 09-23-2010, 11:02 AM
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Default RE: Need expert advice on thin antena vs reception performance

You could also  get with the manuefacturer of the equipmenmt  .

Case in point ,I have a Cyber Sting Ray at first out of the box it had a lousy range of about 15 ft and a few ft underwater,that had to change so I opened up and in the free spaces was this antenna wire that came out of the casing then around a2 times and then all wound up in the front section was this huge amont of wire!
I unwound it and was astonished that it was at least 3 meters long!WOW for a tiny sub to have such a long antena  and still have crappy reseption why was it that way?

Well what I did was to glue in place small  drops about very few inches onto the antenna in a slow widening coil outward until as much of the wire was used up then the rest coiling back,it took a while believe me and I thaught this beter work.
It did and made the reseption increase 5 fold at least and made that sub a blast!

So like Satalites they transmit at a very low power and interplanetary probes even less at huge distances there signal when it  gets to us is so weak that it must be collected by several huge dishes called  antena arrays arrays and the bosted tremendusely just to get the data.

To transmit comands to control those probes takes very powerfull microwwave TX and so that the small antenna on the probe which is usualy a parbolic dish can in turn collect the weakend signal.

So "if you can" manage to extend the antenna on certain kinds of radios like the 27 mghrz or even the ones I use on the bigger subs it does help quite a bit,I even extendid the TX unit on the Telemitry system on bourd the the sub and that realy made a difference while underwater .

I would not increase the TXs power out put unless you knew exactly what your doing and only the maker of the equipment or electronicks person specialized in that equipment  to do the mods ,it could get expensive.
Also arrent some subs useing very long ,even miles long towed antennas,I know the Akula class does.

Just do it simply by increasing the antennas lenght until you get a desired descent reseption .


Dave   
Old 09-23-2010, 11:06 AM
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Default RE: Need expert advice on thin antena vs reception performance

I know that many radio manuefacturers say don't extend the RXs antenna but hay it works .

Dave
Old 09-23-2010, 05:59 PM
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Default RE: Need expert advice on thin antena vs reception performance

Probably when the transmitter is overpoweringly strong, it does not matter what congiguration the receiver antenna is. Reason I say that is, lots of marine hobbyist told me they have good reception by just wrapping the long antenna wire repeatedly onto the receiver. Others said line it along side of the ship is good enough. Some said just wrap it up along the mast. All seem to defy the theory of standard ' half wave ' setup with 1/4 wave in air & 1/4 wave in ground or in contact with a ' conducting' surface.
I have an extra question here though. Does anybody have experience of the resultant effect of a kinked antenna ( like bent 45 or 90 degrees at the end). Reason is that I am thinking about tracing my antenna from the cable to go 90 degrees up with the periscope on the U-boot Vll
Old 09-23-2010, 06:40 PM
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Default RE: Need expert advice on thin antena vs reception performance



The antenna for the receiver is usually cut from the manufacturer in a quarter wavelength size. What does this mean? it means that the length of the antenna is as long as 1/4 of the wavelength of the rf signal. Receiver antenna length is not as critical as the transmitter length is. This is why that always say to extend the transmitter antenna all the way out. If you want to extend the receiver antenna, you want to figure what your frequencies wavelength is. the formula is wavelegth = velocity(speed of light)/frequency

So without dropping down into channels themselves, a 75 mhz radio's basic wavelength is 300,000 m/s / 75,000,000 hzor simplier 300/75 and you get 4 meters! take this times 3.28 and you get 13.12 feet for a full wave receive antenna. divide this by 4 to get the quarter wave antenna, 2 for half wave, etc..... If you want to figure it out for your radio channel, get the frequency of that channel (i.e. 75.XXX mhz) and do the math with that number instead.

Your receive reception is best when done in quartes from the full wave, of course the best is a full wave antenna.

Do not place your antenna in carbon fiber, the carbon will degrade your reception, same with polycarb tube, but to a lesser extent. make sure you seal the ends too, because any water that gets in between the insulation and the antenna wirewill reduce performance also, depending on the mineral content thin shouldn't a make a huge difference

Based on this simple math, it isn't hard to believe that they had a three meter antenna in that model Dave.

Tim

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Old 09-23-2010, 06:42 PM
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Default RE: Need expert advice on thin antena vs reception performance

Dave,

The miles long antenna towed by the subs are actually sonar hydophone arrays, they are streamed out far enough behind the boat to eliminate own ship noise. Subs also have floating radio wires that are used, but they are only typically streamed out a couple of hundred feet for radio reception.

Tim
Old 09-23-2010, 07:17 PM
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Default RE: Need expert advice on thin antena vs reception performance

For the sake of education why dont you explain how coiling (as some people do) effects reciever reception.
Old 09-23-2010, 07:45 PM
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Default RE: Need expert advice on thin antena vs reception performance

ok,

Here is the short of that one. coiling, especially in short tight coils, in effect reduces the length of the antenna and the signal strength being received respectively, thus reducing the range. Dave saw this in his Cyber Sting ray where the antenna was coiled up in the front of the boat. Yes it is one way to fit the antenna in the container, but it negatively impacts performance. The people who are haveing good reception that way are lucky IMHO. They could just as easily have bad reception.

Another fun fact is that the lower the frequency, the better the signal penetrates water, depending on mineral content of course. Everybody knows (or should know) that 2.4 ghz will not penetrate water more than a few inches. the lower freqs will penterate water further with 27 mhz going the deepest. I have succesfully operated my 75 mhz set down as deep as 11 feet. Kevin, you saw this when we had the club meet at the East Providence High school and Thresher went to the bottom of the deep end and still worked.The chemical and mineral content in the water willaffect the reception as well, the higher the content the worse the reception.

I do not have any experiance or data about a kinked antenna, but Inoticed that alot of the 2.4 Ghz sets have this config.

Tim
Old 09-23-2010, 08:17 PM
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Default RE: Need expert advice on thin antena vs reception performance

Actually the antenna in them is not kinked. That is merely a housing for the real antenna which fits loosely inside. I believe this is done because 2.4 requires a good line of site. I heard from some gentlemen that run gas race boats that they have issues with the system because the engines interfere with them and if you dont place them above the water line for a good line of site signal reception can be iffy.
Coiling 3 ft of antenna in one spot is a waste of material and pretty poor design. It is like they knew enough to try and do a half wavelength then couldnt figure out where to put it.

Do you have any experience with base loaded antennas?
Old 09-23-2010, 08:21 PM
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Default RE: Need expert advice on thin antena vs reception performance

The thing I am worrying is that the kink might shift or start a new resonance point on the 1/4 wave antenna
Without the kinked vertical section, then after the Vll sub's front protective cable completely submerged, I do not have any antenna above water left for the bare minimum incoming transmitter wave. The problem in my case, the coning tower is where my water pump motor is and putting the antenna on it will guarantee perfect overwhelming interference. Other location ruin the look of Vll big time .[]
Old 09-24-2010, 02:26 AM
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Default RE: Need expert advice on thin antena vs reception performance

To All,

The original poster said that he desired expert advice. I make no claim to be any kind of expert but I do have over 50 years experience as an amateur radio operator (call sign K8XW) during which I have played with a lot of antenna designs.

First of all a coil has the effect of lengthening an antenna not shortening it. Perhaps that is what the other gentleman was saying in another way - I do not know. I used to run a lot of mobile radio out of my car especially on the 4 MHz band commonly called 75 meters. 75 meters because that's the approximate wavelength of radiation at that frequency in air. If I wanted to run a 1/4 wavelength end-fed antenna it would have to be theoretically 75/4 or 18.75 meters long or about 61.5 feet. Not too practical. So how did we do it? We take an 8 foot long antenna (the length is chosen for practical considerations more than anything else) and feed the energy from the transmitter through a coil and then to the 8' antenna. Such an arrangement is called a base loaded antenna. The coil is designed to add an inductive reactance to cancel out the capacitive reactance of the antenna thus making the overall system more or less resonant at 75 MHz. So bottom line - coiling an antenna serves to raise the resonant frequency of the antenna taken as a whole.

Secondly, all of the above calculation are a total waste of time when we submerge the antenna in water. I guarantee you that the various radio control manufacturers design their antennas based on the antenna being, more or less, in the air where the above calculations make sense, more or less. When we submerge the antenna, we then have to take into consideration something called velocity factor. Velocity factor is defined as the ratio of the speed of light in a vacuum to the speed of light in any other medium, in our case - water. A factor of 75% for water is not unreasonable. This means that if I go through all the calculations and determine that the antenna should be 4 feet long (just to pick a number out of the air), if I then wish to submerge this antenna in water it should be only 3 feet long ( 4 feet X 75%).

So bottom line of the bottom line:

1) Try to keep the antenna straight as much as possible.

2) Try not to run it next to any conductor as the conductor will detune the antenna screwing up the resonant frequency even further.

3) Run the antenna in the dry or wet - it doesn't matter. Putting the antenna inside a dry WTC is the commonly accepted way of doing things. I run all of my antennas outside in the wet with no ill effects (so far!).

4) Be grateful that these systems work at all.

Dan
Old 09-24-2010, 11:14 AM
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Default RE: Need expert advice on thin antena vs reception performance

Yes I agree with the non expert completly ,no matter how much calcultion is expendid it still boils down to plain old experiance in what works best and what doesnot in the real   inviorment .Unfortunatly if i wanted to make the antenna that high out of the sub it would be very ungainly and cause more drag than its worth .Not practicle ,that is very important in this hobby,practacality.
I also run my antenna out in the wet full lenght going around the casing just laid in,seal the ends very important,copper wire dose not like water very much.

I can run to some good depths of 25 to 30 ft in the Den which has some of the best water in the world thankfuly vertualy no minrals excetra to bother with.
The only place I was able to go deeper was in Lake Tahoe back in the 80s I don't know how it is now but fenominal reseption was possible on a simple AM radio and a depth of some 60 ft was done with a Robbe Seawoltf,great little sub!

I also have tried base loaded antennas made for the RXs and found that the reseption was worse by far when submerged,there might be some custom setup that would due?

As for the Cyber Ray the coiling out into its wide Rays wings did the trick ,kind of like a parabolic dish ,put one of those on a scale sub ouch,super drag.

I also use PCM radios with great sucsses ,I get very little to no glitching and as you all know I realy pack that fish with pumps,motors,solinoids and miccrow switches,realativly little electronics I like to keep the high voltage away from the radio pack as much as possible,the reason I like PCM is that one gets imediate notice if your running in an area of poor reseption and by running with the nave lights on the servo that turns on that system twitches letting me know to pull up,works very nicely no need to wait till its gone deeper and then your only in worse shape and might lose her completly,of course I have built in failsafes as well,but with the lighting system it uses less resources to let me know to get up fast.
The Futaba PCM 9 CAP 32 channel Robotic radio is my maine radio and has proven to be superb along with JR 10 ch PCMs give me plenty of flexability and great underwater performance with the huge host of programing and multy model capabilities are the ticket,they cost more but you get what you pay for.

Dave


 
Old 09-24-2010, 01:03 PM
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Default RE: Need expert advice on thin antena vs reception performance

bottom line - coiling an antenna serves to raise the resonant frequency of the antenna taken as a whole.
Since lower freqency resonantes with longer antenna. Using a shorter loaded antenna to replace the regular length , should mean ' coiling an antenna serves to LOWER the resonant frequency of the antenna taken as a whole ' ??

Old 09-24-2010, 01:38 PM
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Old 09-24-2010, 01:42 PM
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Default RE: Need expert advice on thin antena vs reception performance

You're absolutely right, I said it backwards. I was thinking in terms of wavelength not frequency. The eight foot antenna, quarter wave resonant at about 30MHz, will be resonant at about 4MHz with a proper base loading coil. Said another way (as you said) coiling the antenna should allow operation at a lower frequency. I wrote this late at night and was just too tired. What I said was the coil has the effect of lengthening the antenna in that a shorter antenna can act like it was a longer antenna - I still think that's correct but it may have been hard to follow - sorry for the confusion.

Having said that and totally accepting your correction, I still maintain that the environment in which we operate with radio frequency noise sources everywhere close to the antenna and operating underwater leads me to believe that if you're going to spend money anywhere in the radio data chain it's best spent on using the best receiver your budget allows and let the antenna be placed wherever it'll fit. Coiling the antenna can lead to other undesirable effects like a reduced frequency bandwidth response but on the whole there are just too many other variables involved to try to sort out the whole mess.

Thanks for the correction,

Dan
Old 09-24-2010, 03:02 PM
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Default RE: Need expert advice on thin antena vs reception performance

yup sounds about right and one can't genralize with these radios,the mass produced cheaper ones like the ones found on the Cyber Shark are not the best it could be although there getting better with less power cunsuption.The sub however is great it will run at least 2 hours at full on can you believe this 4 duble A Duracells ,unbelievable!.

I remember now that once I coiled a long standard RX antenna around the round axis of a casing to see if it maid any difference ,it was neglegable.

Another helpfull thing to do  to reduce radio interferance is to seal the battery terminals as electrolosis and intercunducting currents in the free flood area could play a role in reseption quality and instaling choke coils on high voltage and low voltage esential circuits along with some degree of shielding  and always seal any circuits that are in the wet as feedback in the water is greatly incresed that can ruin your day. 

Dave
Old 09-24-2010, 03:11 PM
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Default RE: Need expert advice on thin antena vs reception performance

I find that fish like to shase certain subs of mine they like the ones that have low electromagnetic signatures and noise,the others they scamper away.The ones that chase me I pull crazy Ivans and they play around with it,lots of fun,I find that crocks don't care about the sub to much.[:-]
Never even had one chase one just the boat I was in and he fled right away,they  seem to know what is a machine and what is not.


Dave
Old 09-24-2010, 03:13 PM
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Default RE: Need expert advice on thin antena vs reception performance

Love that babe she would make a great # 1  mama sita!

Dave
Old 09-24-2010, 08:19 PM
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Default RE: Need expert advice on thin antena vs reception performance

"I wrote this late at night and was just too tired. What I said was the coil has the effect of lengthening the antenna in that a shorter antenna can act like it was a longer antenna "

These thing do happen. I am qualified to tell you because I have uploaded more than 50 Youtube RC videos and answered comments. Often I typed in words that I meaned something else and most of the time I caught it in time but still sometimes I didn't. By the way, Canton is a nice area. I used to drive around there & then ended up barbecuing on Grosse Ise, before that Sept 11.
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Old 09-25-2010, 01:38 AM
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Default RE: Need expert advice on thin antena vs reception performance

Something has been bothering me all day and I just figured out what it was. I am only adding this to clear up any confusion which some may have due to my less than clear original explanation on the effects of coils in conjunction with antennas.

I originally said that a coil serves to lengthen an antenna - that is true in that a coil allows a short antenna to act as if it was a longer antenna.
On the other hand, pl_09 said that a coil acts to shorten an antenna - this is also true in that a long antenna can be shortened by adding a coil.

I now believe both of us were saying the same thing but from a different perspective.

I hope this clears up any misunderstanding.

Sorry for this mess,

Dan

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