What length wire for which frequency?

Howdy again... I am trying to make myself some cheapo twig aerials for my trackers using SMA connectors.

I have a BRB 900, a Telemega and a Eggfinder.

What length wire do i need for each, for TX and for RX (1/4 and 1/2 wave???).

Also, can someone point out a nice idiot friendly website that explains all this.

Thanks

S1

Wavelength in meters will be 300/Freq(Mhz).

Take 1/2 or 1/4 of that for half wave or quarter wave.

Then multiply by velocity factor. Not sure what you mean by 'twig aerials', but for a dipole or ground plane, 70% is about right as a velocity factor.

300 Mhz = 1 meter (39.37"), so a 1/4 wave antenna is 300/frequency * 39.37/4 inches, or 300/frequency * 250 mm. A half-wave antenna is twice as long.

Well, wavelength = velocity/frequency, where velocity is basically the speed of light, or 300,000,000 m/s.

The BRB900 and Eggfinder are probably around 915MHz, and the Telemega is in the 70cm Ham band (I won't ask...) or 433MHz.

So, my flimsy math says:

300,000,000 / 915,000,000 = 0.328 meters. 1/4 wave would be 0.082 meters, or 3.23 inches for the 915MHz devices.

300,000,000 / 433,000,000 = 0.693 meters. 1/4 wave would be 0.173 meters, or 6.81 inches for the 433MHz devices.

A 1/2 wave antenna is generally a dipole, often center-fed, with two opposing elements, one "driven" and one "counterpoise." A 1/4 wave antenna is more likely appropriate for these devices, unless you've got lots of space. A 1/4 wave antenna relies on the ground plane of the circuit board as its counterpoise. There's probably slightly more gain and even radiation pattern from the 1/2 wave, but again, the 1/4 wave will be more practical.

You can help the antenna efficiency by making sure that it's straight, with smooth ends. Jagged, sharp cuts do affect the radiation.

I can't come up with a single, best Web site to describe this, but if you're interested, see whether a friendly ham or nearby library has the ARRL Handbook (paper or pdf).

Have fun.

Mark

Thank you...
Yeah, I need to read the books that i have, i bought the recent ARRL for Tech level and really need to study for the test before i can use that Telemega. That's why I have the two at 900.
Asking on here was kinda lazy, but I knew that there would be peeps who knew the answer!

Salvage-1 said:

Asking on here was kinda lazy, but I knew that there would be peeps who knew the answer!

Click to expand...

Nah - you shouldn't hesitate to ask. My comments were tongue-in-cheek, and I should have kept them to myself.

Mark

If the antenna is connected directly to the transmitter board without any coax or other transmission line you can ignore the velocity factor. It's going to be in the high 90%'s anyway. You'll get more bang for the buck by improving the receiver's antenna. In the case of the Eggfinder, I've had customers get over 20,000' with the simple wire antenna on the transmitter and a half-wave dipole rubber duckie on the receiver.

You generally can't do any antenna tuning with the rubber duckie antennas anyway, so the best rule of thumb is to keep any extension cables as short as possible, while leaving enough slack in them so that they can't be strained during deployment movements. A small loop zip-tied to the shock cord near the bulkplate is usually sufficient.

Well, we can agree to disagree. Everything that I have read indicates that a piece of bare wire used as an antenna will have a velocity factor > 95%. If there's no coax or other transmission line to get in the way then that's about as close as you're gonna get to ideal. Putting the shrink-wrap on the bare antenna reduces the velocity factor by a little bit (it turns it essentially into an insulated solid-core wire), but it's not very significant. The "theoretical" length for a 915 MHz antenna should be 82 mm, in the Eggfinder manual I recommend cutting it to 80 mm in deference to those small unknown variables.

Every Eggfinder installation that I've seen with a coax extension has used a rubber duckie, and since they can't be tuned it's kind of a moot point. The issue is not so much as if it's as good as it could possibly be, but if it's good enough. Judging from the reports I have from the field, it is.

W7AMI said:

I disagree. The velocity factor on the antenna wire is important otherwise your antenna will be too long electrically. The only time the velocity factor of the coax feed line matters is when you are building things like phasing lines to combine two (or more) antennas together or building 1/4 wave matching sections. Something very few people here would need to do. The velocity factor for most solid dielectric coax in around 65%.

Click to expand...

The ideal antenna is a purely resistive load and should have negligible inductance and capacitance. As such the electrical length should be very close to the same as the speed of light in air resulting in a velocity factor very close to 1. The author of the calculator you used either doesn't understand the physics of antennas or has not adequately described what he is doing. Perhaps he is using a LC matching circuit to resonate a shorter than 1/4 antenna element.

https://en.wikipedia.org/wiki/Velocity_factor

https://en.wikipedia.org/wiki/Antenna_(radio)

Bob

And that reference simple confirms what everyone else in this thread has posted....

1.) To estimate the length of a half wave dipole the simple formulae given below can be used: Length (metres) = 150 x A / frequency in MHz or Length (inches) = 5905 x A / frequency in MHz

2.) The actual length of an antenna is slightly less than the physical length by typically ~2%-4% due to the finite thickness of the radiating element.

3.) It is always best to make any prototype antenna slightly longer than the calculations might indicate. This needs to be done because changes in the thickness of wire being used etc. may alter the length slightly and it is better to make it slightly too long than too short so that it can be trimmed so that it resonates on the right frequency. It is best to trim the antenna length in small steps because the wire or tube cannot be replaced very easily once it has been removed.

From the chart in your reference, if you made a 1/4 wave antenna for a 915 MHZ transmitter from 1/16" diameter wire, the wavelength to diameter ratio is (12.91"/0.0625") = 206 which means that the physical antenna would be 96.6% of the free space wavelength which is calculated as 300 x 10^6 m/s /915 x10^6 /s = 0.3279 m = 12.91" so the 1/4 vertical should be ~ 0.966 x (12.91"/4) = 0.966 x 3.227" = 3.112" which is ~1/8" shorter than the free space wavelength.

If you want to calculate the length directly in inches, the equation is VF x 11810"/us / freq Mhz) x (antenna element in wavelength) = 0.966 x (11810/915) x (1/4) = 0.966 x 12.91 x 0.25 = 0.966 x 3.227 = 3.112"

When you go above 1 Ghz, the wavelengths get short. 2.45 Ghz is ~4" (10 cm) so a 1/4 dipole is only ~ 1" long. If you made a 1/16" thick antenna, the wavelength to diameter ratio is ~ 64 and the correction factor exceeds 4%. You can make an antenna with greater bandwidth by making the elements thicker and shorter, but vertical antennas are already have good bandwidth, and neglecting the velocity factor for 915 Mhz transmitters shouldn't effect range greatly, however if you make an antenna for a transmitter, you should have the test equipment to check the performance because if it does not emit efficiently you can damage the transmitter. Trying to save a few dollar by making your own antenna on a radio that cost $50 or so can be expensive if you burn up the power transistor.

References.

https://en.wikipedia.org/wiki/Antenna_(radio)

https://en.wikipedia.org/wiki/Antenna_measurement

https://en.wikipedia.org/wiki/LC_circuit

Bob

I am sorry for any confusion I caused.

No need to be sorry. Not every calculator or website has 100% correct information, and it's not always obvious. The calculator you used would probable work well enough at these frequencies because 1/4 wave verticals are inherently broadband. Take the back off your phone and look for the copper tabs or conductive paint. That's the phone antenna and the GPS antenna. Some work ok, and others work very marginally. I have several extended battery back. With one the GPS works great but the phone signal is poor. The other is just the opposite. The one that came with the phone with the standard battery works well with the phone and GPS....

Bob

cerving said:

Well, we can agree to disagree. Everything that I have read indicates that a piece of bare wire used as an antenna will have a velocity factor > 95%. If there's no coax or other transmission line to get in the way then that's about as close as you're gonna get to ideal. Putting the shrink-wrap on the bare antenna reduces the velocity factor by a little bit (it turns it essentially into an insulated solid-core wire), but it's not very significant. The "theoretical" length for a 915 MHz antenna should be 82 mm, in the Eggfinder manual I recommend cutting it to 80 mm in deference to those small unknown variables.

Every Eggfinder installation that I've seen with a coax extension has used a rubber duckie, and since they can't be tuned it's kind of a moot point. The issue is not so much as if it's as good as it could possibly be, but if it's good enough. Judging from the reports I have from the field, it is.

Click to expand...

Hi Cris,
I just finished cutting my first eggfinder 915 Mhz Tx antenna .. I cut it at 82mm from the edge of the PC board and did solder all the way to the edge. I just read your post here and thought I'd ask if you think I should cut it down 2mm to 80mm per your manual.?
Thanks,
Kevin

I was watching a PBS NOVA program on fractals, and antenna lengths were discussed... Don't know if it helps you or not, but it's something to think about.

I read once about tuning 1/4 wave wire antennas by trimming them down, measuring the power draw the radio is using (or rssi if available), then trimming a little more and repeating.
I can't find the reference anymore and was wondering if anyone here knows the process. (ie when to stop trimming)
Would something similar work here?
would this process give any benefits?
Could this process even work if you are using frequency hopping radio modules?

A simple wire antenna has maybe a 0 dB gain, you might get a tad bit more if you went to the trouble of tuning it. Better to get an external antenna for the receiver instead.

https://www.rcgroups.com/forums/showpost.php?p=13840440&postcount=10
I found the reference to tuning antennas.
I have ordered connectors for my eggfinders and plan on using external antennas on most of them, especially the receivers.
Sometimes space constraints make your supplied antennas a better option for the transmitter and I figured that as I will have a few spares would this process optimise their signal?

Per the HAM radio license manual, page 4-12, and attention to questions T9A08 and T9A09:

Length of 1/2 wavelength dipole in feet = 468 / frequency in MHz
Length of 1/4 wavelength dipole in feet = 234 / frequency in MHz

as was said, the antenna acts longer electrically than physically. One can connect an antenna analyzer at your local conditions to find a resonant frequency of your antenna that would provide the best reception