Antenna question?

[ReddyWatts]

What are the receiver antenna wire lengths for the different bands 75 mhz and 2.4 gig?

Do you figure the wave length in inches and divide it by 1/8, 1/4, 1/2 etc....?

Is it total length measured from receiver or the amount outside the hull?

http://www.onlineconversion.com/frequency_wavelength.htm

[icelert]

One of my other hobbies a long time ago was amateur radio so I think I can help you here(I hope).

Antennas are made to match the wavelength of a particular frequency. The lower the band frequency the longer the antenna will be.
For example, 27mhz is the 11 meter band. A full wavelength antenna will be 11 meters in length. That is too large for most applications so smaller antennas are made, 1/2 wavelength is 6.5 meters, 1/4 wavelength is 3.25 meters etc. Total wavelength is measured from the end of the coaxial wire exiting the receiver.

Do you figure the wave length in inches and divide it by 1/8, 1/4, 1/2 etc....?
Yes. You can use either metric or standard dimensions.

That link to the online conversion website is a great tool for calculating antenna wavelength dimensions!

Is it total length measured from receiver or the amount outside the hull?
I just read on one of the forums recently that the Spektrum 2.4 ghz antenna has a bump in the wire exiting the rx about an inch from the end. That bump is the end of the coaxial wire and the last inch is the antenna. That is why you don't want to cut back that wire!
One more thing about antennas, especially 2.4 ghz, make sure they are oriented in a vertical fashion. That will give the best receptive performance from the antenna. Laying it horizontally inside a hull may work, but the efficency of the antenna will be compromised. A CF hull will shield RF frequencies so make sure and antenna is externally placed if a hull is entirely CF.

In a nutshell 2.4 ghz is a UHF frequency of .13 meters. A full wavelength antenna would be about 5 inches long. The Spektrum rx's are probably 1/4 wavelength antennas of 1.23 inches.
75 mhz is a VHF frequency of 4 meters. A full wavelength antenna is 157.4 inches so a 1/16 wavelength antenna of 9.8 inches is workable.

Bryan

[ReddyWatts]

Would this be correct?

A 75mhz receiver and the wire are cut to 1/8 wave length of 19.68 inches, so if you use one of the Gundert Whip Antennas - 20, 28 or 45cm should you cut the wire and antenna total length is equal to 1/8 wave length, since it is not shielded?

Why the three different lengths of antennas?

http://www.offshoreelectrics.com/pro...nd-whip&cat=73

75mhz radio system
19.68 (-) 7.87 inches = 11.81 inches of wire plus 20cm antenna
19.68 (-) 11.02 inches = 8.66 inches of wire plus 28cm antenna
19.68 (-) 17.72 inches = 1.96 inches of wire plus 45cm antenna

http://bluefive.pair.com/inches_to_cm_to_inches.htm

[icelert]

If the wires coming out of the 75 mhz rx are not shielded, and all of the wire exiting the rx is antenna, then your answer would be yes.

[egneg]

It has been a long time (30+ years) but I seem to remember that electrical length of an antenna is NOT the same as the physical length - capacitance and inductance are used to adjust the electrical length.

[ReddyWatts]

So which Gundert antenna should you use for 75mhz and does the length of wire from the receiver matter for best reception? It will need to be shortened. hmmmmm......................

It seems like you should cut it to the 1/8 wave length and hope for the best.

[icelert]

Well far be it for me to tell you which is best, you have a depth of knowledge on this forum that is greatly appreciated.
But if it were my setup I would go with a Bru-Line antenna and fish my rx antenna through it. This will preserve the integrity of your antenna wire and make it easy to swap it out to other boats later on. The Bru-Line is long enough that it is probably well suited for 72-75 mhz rx's. I paint mine flat black to make it less visible and I find that the antenna will flex more than the Gundert thereby less likely to transfer stress where it is drilled through the hull. On my boats I use Spektrum rx and cut the Bru-line to about 3 inches.

[ReddyWatts]

I like your suggestion icelert.

Still wonder why the Gundert comes in 3 lengths.

Are they different gain?

Is it just cosmetic or different frequencies?

Should we shield the 75mhz receiver wire up to the Gundert antenna?

[icelert]

No idea. If I had to guess it would be that the Germans use different frequencies for their r/c's. At their website the antennas range from 21cm-51cm.

Rx antennas are not as crucial in matching to a frequency or wavelength as transmitters are, but if they are oriented in a vertical fashion they stand the best chance to receive the signal from the tx.

[G Doggett]

Very interesting topic.
You are right Ice, different countries use different frequencies for rc use.
Down here in NZ we cannot use 75 mhz for instance.
Obviously shortening an antenna length is a no no, but would lengthening it have much of a detrimental effect ?
I'm thinking of a situation where you have a Spectrum radio with its very short antenna which will not reach to the 'outside' of a boat.
Would adding an extension to the end of the original wire to take outside the hull couse more harm than good ?
I'm considering switching to Spectrum but probably coz I'm old fashioned I like to see an antenna sticking up high in the air !!
Graham.

[icelert]

Graham,
What is your boat contructed of? If it is other than CF you could run it inside the hull hopefully in a vertical fashion without adding to its length.
Another possiblity is to cut a Gundert antenna down to a very short length(1-1.5 inches) and solder a blade connector to the end of the Spektrum antenna. I did this on a boat last year with success. Then I glued a red colored bead from my wife's bead collection to the clipped off end of the Gundert.

[G Doggett]

No CF on my hulls Ice, just good old fibreglass or plywood.
I would like to keep the existing external antennas that are installed on the fleet but I guess I could shorten them like you did.
This is still increasing the over all length, as the Spectrum antenna is not being shortened to compensate for the Gundert length, but I presume you have had no problems as a result of doing this.
Graham.

[ReddyWatts]

What i have learned is that 2.4 gig radios can use an antenna that is about 1.25 inches equal to 1/4 wave length, so 5 inches equals one full wave length. I guess you could run some distance tests. Take the boat outside and get someone to walk the transmitter down the street and see if different antenna setups hurt your range? It would be good to know how far your transmitter will reach, before you run it. That is a test we probably should perform on all new builds. We always assume the radio system is ok.

[icelert]

Yes I have done that without any problems. Up here in the Pacific Northwest we do lots of club racing with many many boats and I have seen others have their share of radio related worries but my Spektrum worked great w/ the shortened Gundert antenna. I switched this year to the shortened Bru-Line because I added other boats to my fleet and I wanted to be able to switch rx's if it became necessary.
Like I mentioned earlier I would never think of doing this to the tx because getting the signal out is more crucial. In ham radio we used an SWR(standing wave ratio) bridge, a meter hooked between the tx and the antenna. It showed what the efficiency of the signal was(ie how well tuned) coming out of the tx to the antenna.

[G Doggett]

Ok , that make sense Reddy.
So if the original Spectrum or Futaba Fast antenna is only 1.25 inches long I could attach that to the base of an external antenna that is 3.75 inches high to give me the 5 inches full wave length.
I like the sound of that.
Would the received signal be superior because it was full wave instead of 1/4 wave, or does that not make any difference ?
Graham.