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View Full Version : How are frequencies and crystals handled at the field?


Jeremy Z
10-17-2005, 11:06 PM
I'm thinking of buying a radio to go with my second plane. I will possibly retrofit my first plane to work with this radio too.

I'm trying to decide if it is worth the money to get a synthesized transmitter or not. There are about 50 frequencies available to the RC pilot, right? How is this handled? Does each pilot have all 50 sets of crystals in his toolbox? Is there one complete set owned by each club or kept at the field? That would be a pain in the neck, and would probably mean it's worth the extra $50 to get a computer radio with Spectra module.

I just don't want to run into a situation where I go to the field, ready to fly, and have to wait in line for the frequency for which I have crystals to open up.

Jeremy

rcers
10-17-2005, 11:16 PM
How is this handled? Does each pilot have all 50 sets of crystals in his toolbox?

No the club does not handle anything in relation to frequency except for control. Only one radio on per channel. So you have a board that you check for your frequency before turning on your radio. You usually put your AMA card in a slot with your freqency on it and go fly. Sometimes you take a "pin" off the board to show you have that frequency in use.

The spectra modules just allow you to fly on another frequency if your is in use, but you will have to supply the crystal for the radio receiver.

Mike

qban_flyer
10-18-2005, 12:08 AM
There are about 50 frequencies available to the RC pilot, right? How is this handled? Does each pilot have all 50 sets of crystals in his toolbox? Is there one complete set owned by each club or kept at the field? That would be a pain in the neck, and would probably mean it's worth the extra $50 to get a computer radio with Spectra module.

Jeremy

Very carefully! LOL.

Kidding aside, current radios are tuned to operate within a very narrow band. While we can switch crystals on the receivers (some brands will accept crystals on all frequencies while others have a Low Band & a High Band restriction) transmitters are limited to a change of no more than two (2) channels on either side of the one that it came with from the factory.

Lets say you bought one on channel 36. You can change the TX crystal to go down as far as 34 or as high as 38, anything other than that will spell trouble for the entire set up.

Enter Futaba and Hitec with their computerized radios sporting synthesized modules for their TXs. Now all you have to do is to take two or three models with you to the field and one single TX. Lets say that each plane is on a different channel than the others. When you are ready to fly model A, you change the 'model' in your computerized TX to model A, and make sure your 'module' is set to the particular channel used with that plane. The same applies to the other planes you have taken with you to the field that day.

One advantage of, lets say, the Hitec Optic Six is that it has a 10 model memory and a Spectra module. It means that you can store 10 different models in its memory and can use each one of them with whatever channel you may wish.

Another advantage of a computerized radio is that it 'remembers' the trim settings for each model. Once the plane is trmmed to your satisfaction, the trim settings are stored for that particular model in the TX memory bank.

Yes, a computerized radio is desirable if you are seriously considering staying in this hobby for the long haul. They have many more features in addition to the ones already mentioned. The things you can achieve with a computerized system are without boundaries.

I strongly recommend to newcomers serious about the hobby to get at least a six channel computerized radio. It will not become obsolete when they are ready to move up to bigger, better and more expensive flying things.

Rugar
10-18-2005, 03:52 AM
One advantage of, lets say, the Hitec Optic Six is that it has a 10 model memory and a Spectra module. It means that you can store 10 different models in its memory and can use each one of them with whatever channel you may wish.


Actually the Optic 6 has a 8 model memory. I wish it was 10! ;)

I have a Optic 6 with the Spectra module. I have two differant RX crystals on differant freq. for each brand of RX that I use. (I mainly use Bergs) I go to the field with a choice of using two Freq. so if one is being used I can change to my other one by changing the crystal in the RX and the channel on my TX. I can change over in less then a min.

Gerald

qban_flyer
10-18-2005, 04:06 AM
Actually the Optic 6 has a 8 model memory. I wish it was 10! ;)

I have a Optic 6 with the Spectra module. I have two differant RX crystals on differant freq. for each brand of RX that I use. (I mainly use Bergs) I go to the field with a choice of using two Freq. so if one is being used I can change to my other one by changing the crystal in the RX and the channel on my TX. I can change over in less then a min.

Gerald

I must have been thinking of the Futaba 7CAP, though 8 model memory is more than enough for one TX.

My two Futaba 8 "Super" TXs handle 8 models each though I can extend that to 32 models by inserting a CAMPAC module on the front port, but who wants to have 32 models on one TX?

Although the Optic 6 has one channel less than the Eclipse, IMHO, I found it to be a more user friendly TX than the Eclipse.

BTW, the Spectra module works perfectly with the futaba TXs. I have two Spectras and have found that my antenna collapsed range using the Spectra is approximately 75' further than with Futaba's own module! Talk about something really odd.

Rugar
10-18-2005, 04:25 AM
I must have been thinking of the Futaba 7CAP, though 8 model memory is more than enough for one TX.

I wish I had more! I have 8 RTR planes right now with several more kits to put together. Im debating on either getting another brand TX that will hold more models in memory, or getting another Optic.

My two Futaba 8 "Super" TXs handle 8 models each though I can extend that to 32 models by inserting a CAMPAC module on the front port, but who wants to have 32 models on one TX?

ME! :D

Although the Optic 6 has one channel less than the Eclipse, IMHO, I found it to be a more user friendly TX than the Eclipse.

I agree. Its a little easier to program, but the main thing for me is that it just feels alot and more natural and has a better feel in my hands.

Gerald

qban_flyer
10-18-2005, 04:46 AM
I wish I had more! I have 8 RTR planes right now with several more kits to put together. Im debating on either getting another brand TX that will hold more models in memory, or getting another Optic.


Gerald

The very reason I have two Futaba 8 "Super" TXs and one Hitec Flash 5 "System X". It allows me to keep 21 of my most used 46 e-planes on memory.

Stick with Hitec, it's a very good and reliable brand at very good prices.

r_kopka
10-18-2005, 10:04 AM
Very carefully! LOL.

Kidding aside, current radios are tuned to operate within a very narrow band. While we can switch crystals on the receivers (some brands will accept crystals on all frequencies while others have a Low Band & a High Band restriction) transmitters are limited to a change of no more than two (2) channels on either side of the one that it came with from the factory.

Lets say you bought one on channel 36. You can change the TX crystal to go down as far as 34 or as high as 38, anything other than that will spell trouble for the entire set up. Are the radios in the US(?) so different ? Here(Austria-Europe) I read nothing like that in the radio manuals. We can choose between all channels of one band. 60-80 35.000-35.200MHz.

I had 24 models with my FX18 and small CAMPAC which was not enough. Now I have 61 :-)

RK

slipstick
10-18-2005, 10:49 AM
In the US it is a LEGAL requirement that users do not change transmitter crystals AT ALL. Even changing up or down one or two channels is against the FCC regulations and if you're going to break the law you may as well go the whole hog. There's no TECHNICAL reason why you can't use any channel in a modern Tx (just like users everywhere else in the world do perfectly legally), it's simply against FCC regulations.

Steve

debhicks
10-18-2005, 11:11 AM
If you need more model memory and want to do just about anything with your xmitter check out the Multiplex. Ours only has 36 model memory. I am pretty sure that my garage won't hold more than 10, however we are great stackers. I just draw the line when it comes to the living room:)

I honestly can't see any reason not to encourage the synthisized radio's even as a new beginner purchase if they are sure they want to stay in the hobby. In the long run they will spend less money. :)

The 8103 JR has a new synthisized module available for it too.

qban_flyer
10-18-2005, 05:36 PM
In the US it is a LEGAL requirement that users do not change transmitter crystals AT ALL. Even changing up or down one or two channels is against the FCC regulations and if you're going to break the law you may as well go the whole hog. There's no TECHNICAL reason why you can't use any channel in a modern Tx (just like users everywhere else in the world do perfectly legally), it's simply against FCC regulations.

Steve

I am not an electrical engineer, but have a three friends who are. All three have told me that once you go past two channels on either side of your "narrow" band not only does your range gets reduced, but that your TX can also interfere with operators in the vincinity of the channel you have switched to.

To put it simply (as per their explanation), if you go five channels up or down from your TX's native channel you have just "detuned" it making it a hazard not only to yourself but to everyone present at the field. It's the very reason the FCC has that rule in their book. They don't want anyone getting tempted to use the entire band with one TX as it will make it unsafe for everyone near that TX.

If it were safe to "just change" crystals in order to operate throughout the entire bandwith, why would radio manufacturers be making entire properly tuned RF modules for their expensive TXs?

slipstick
10-18-2005, 10:34 PM
If it were safe to "just change" crystals in order to operate throughout the entire bandwith, why would radio manufacturers be making entire properly tuned RF modules for their expensive TXs?
If it isn't technically safe to change crystals throughout the entire bandwidth why would it be legal (and normal) to do so in every country in the world outside North America ?

BTW the RF modules are mainly to allow changes of BAND. I normally fly on 35MHz but I don't need a complete new Tx when I go to the US, just a 72MHz module. Similarly I can put in a 40Mhz module and be legal for surface craft. Nothing to do with simple channel changes within band.

Steve
(not an electronics engineer these days but most of my training is still valid ;))

qban_flyer
10-18-2005, 11:44 PM
If it isn't technically safe to change crystals throughout the entire bandwidth why would it be legal (and normal) to do so in every country in the world outside North America ?

Steve
(not an electronics engineer these days but most of my training is still valid ;))

Beats me, Steve.

I have to trust what these Electrical Engineers have told me. I have no reason to doubt them, just as they have no reason to mislead me. Two of them have been working at N.I.S.T (formerly know as the U.S. Bureau of Standards and Measurements) in Gaithersburg, MD for well over 25 years.

Jeremy Z
10-18-2005, 11:51 PM
I'm confused. When you get a synthesized transmitter, you can transmit at any frequency within our 72 MHz band, right?
Then, you still need a crystal for which ever channel your receiver is on, right? (unless you have a synthesized receiver)
Am I right to assume that you can use a synthesized transmitter with a crystal receiver, or does one mandate the other?
...and what's this about going outside the channels? The whole point of crystals is that they resonate at a very specific frequency, and that they work in conjunction with the receiver to reject all other frequencies.
Help me understand please.

Jeremy

slipstick
10-19-2005, 10:34 AM
Yes a synthesised transmitter works with single frequency crystal receivers.

But crystal oscillators in transmitters aren't quite that simple. The rest of the transmitter has to be set up correctly so the crystal can work at its intended frequency. That's why you can't use crystals from different manufacturers, the transmitter is set up wrong for them (there's a lot more to a crystal specification than JUST the frequency).

qban flyer's theory is that if you change too far away from the existing channel you either end up transmitting on the wrong frequency or (more likely) with much less power than you should. That used to be true with very old designs but designs improve and every modern transmitter I've tested works perfectly over the whole range.

But that doesn't really matter because it's still against FCC regulations to change transmitter crystals in the USA (unless you have the correct equipment and qualifications to check the results). There are actually some pretty good reasons for these rules but they're mainly based on the conditions and equipment of 20-odd years ago when the rules were first made. So if you're still flying old RC equipment there are good technical reasons for not messing with the crystals, if you're using modern equipment you just shouldn't do it because it ain't allowed. Government regulations don't have to make sense ;).

Steve

Geoff_Gino
10-19-2005, 01:00 PM
Yes a synthesised transmitter works with single frequency crystal receivers.

But crystal oscillators in transmitters aren't quite that simple. The rest of the transmitter has to be set up correctly so the crystal can work at its intended frequency. That's why you can't use crystals from different manufacturers, the transmitter is set up wrong for them (there's a lot more to a crystal specification than JUST the frequency).

qban flyer's theory is that if you change too far away from the existing channel you either end up transmitting on the wrong frequency or (more likely) with much less power than you should. That used to be true with very old designs but designs improve and every modern transmitter I've tested works perfectly over the whole range.

But that doesn't really matter because it's still against FCC regulations to change transmitter crystals in the USA (unless you have the correct equipment and qualifications to check the results). There are actually some pretty good reasons for these rules but they're mainly based on the conditions and equipment of 20-odd years ago when the rules were first made. So if you're still flying old RC equipment there are good technical reasons for not messing with the crystals, if you're using modern equipment you just shouldn't do it because it ain't allowed. Government regulations don't have to make sense ;).

Steve

Hi All

PLEASE take note of Steve's quote here.

If your Tx has crystal (little silver can) in the rear cover it definitely is not synthesised and will NOT be "tuned" for any other slot with a consequent loss in Output power from the Tx. Illegal or not your range is going to diminish rapidly.

Geoff
(Radio Technician)