View Full Version : 2m antenna
07-03-2007, 01:55 AM
i have a newbie question - after getting my license about a year ago, i finally got a radio - my question is i have an old dipole tv antenna (rabbit ears?) with two telescopic antennas on each side - does adjusting the length change the wavelength of the antenna? - if i would pull each of the antennas out to 50 cm on each side, would it make me a 1/2 wavelength, 2m, dipole antenna? #thanks guys - sorry for such an amateur question #http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif
07-03-2007, 03:06 AM
A free space wavelength at 146.5 MHz is 6' 8.6", that would make each half of your 1/2-wave dipole 20.16" if the elements were infinitely thin and there were no insulator at the center (or ends for that matter). Even the little buttons at the end of the elements affect the resonant length of the antenna.
It is difficult to give you an exact answer to your length question; however you can come up with the length experimentally with a VHF VSWR meter and a little knowlege.
Here are the steps to find the correct length for any antenna:
1) Take a VSWR reading at the bottom of the band, make note of the reading.
2) Take a VSWR reading at the top of the band, take note of the reading.
3) If the VSWR at the bottom of the band is lower than at the top of the band, then the antenna is too long. If the VSWR at the top of the band is lower than it is at the bottom of the band then the antenna is too short.
4) If the VSWR is approximately the same at both ends of the band, take a reading at the center of the band. it ought to be close to 1.5:1. Don't expect a vswr much lower than that unless you are using a really lossy transmission line. A properly tuned dipole mounted away from anything which will react with it will have a feed point impedance in the area of 75-Ohms which will translate to a 1.5:1 VSWR. If the VSWR at band-center is far away form the 1.5:1 target you have other problems including things like an insulator not meant for VHF transmitting.
For a first VHF antenna, a 1/4-wave with drooping radials made out of a chassis mount SO-239 and some copper wire. You can find drawings for such an antenna in the ARRL Antenna Book or in various places on-line.
73 DE KAØGKT/7
For your first antenna, build one of these (as suggested by ka0gkt). It's simple and effective and you'll have to build or spend a significant amount more to make much difference. The key to 2 meter work is not as much in antenna gain as it is in line of sight, so height is ever so much more important than gain.
Two Meter Ground Plane (http://www.hamuniverse.com/2metergp.html)
07-03-2007, 10:30 AM
07-03-2007, 11:38 PM
The other posters have given good advice; in particular, GKT has shown how to determine element length.
Your idea should work, unless you are going to run a kilowatt into it. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
This thread is over in Talk & Opinions;
RGR has posted a couple of links to simple antennas; one is a tape-measure yagi & the other is a rabbit-ears yagi.
If you are going to experiment, you need measuring equipment; I recommend the VK5JST VHF/UHF Analyser; you can get full construction details here;
It doesn't get much simpler than that. It's very easy to build.
My version works very well;
07-04-2007, 12:40 AM
thanks for that information http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
07-04-2007, 01:26 AM
VK2TIL mentioned the analyzer. If you are interested in building antennas or just learning more about a commercially manufactured antenna, having a good analyzer is an excellent idea. You can learn much from a good quality VSWR meter as well, however some of the antenna analyzers are quite good and very educational when coupled with a decent antenna text like the ARRL antenna book or some of the tomes written by Bill Orr.
A friend and co-worker; Jim, AD7KT, has the Autek RF-5 VHF Analyst (http://www.autekresearch.com/rf5.htm). We've compared readings taken with the RF-5 to readings taken with a $50K+ Advantest Vector Network Analyzer and the Autek unit is surprisingly accurate especially when you take into account the over 23 dB difference in price http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif (The Autek sells for under $230.00), although admittedly, the VNA can do lots of things the RF-5 can't...like give accurate readings from a few KHz to over 7 GHz, for an Amateur interested in antenna experimentation, in my book, the Autek is great. Martin Jue's antenna analyzers may be just as good, however I haven't had the opportunity to play with one at any length.
Have fun with your antenna experiments, and get on the air.
73 DE KAØGKT/7
07-04-2007, 03:41 AM
This would work however if I remember correctly I believe balanced feed line (twin lead) works better for dipoles. If you get balun it should work though. But you are definitively thinking!
07-04-2007, 05:55 AM
Quote[/b] (KC8RFC @ July 03 2007,15:41)]This would work however if I remember correctly I believe balanced feed line (twin lead) works better for dipoles. If you get balun it should work though. But you are definitively thinking!
the ears i have use ladder line - i guess connecting it to the radio however i would need a balun
07-04-2007, 04:05 PM
I was given some equipment and among that equipment was an antenna tuner, an swr meter, a rf watt meter, and a two way radio tester, at least this is what all of it says - is this what i need to sucessfully test my antennas?
07-04-2007, 04:14 PM
Quote[/b] (KB3NDN @ July 04 2007,09:05)]I was given some equipment and among that equipment was an antenna tuner, an swr meter, a rf watt meter, and a two way radio tester, at least this is what all of it says - is this what i need to sucessfully test my antennas?
The antenna tuner, SWR and watt meters may or may not work - it #depends on whether they were designed for use at HF or VHF/UHF.
I have no idea what a "two way radio tester" is. #I've never seen any such thing - at least, nothing that was called such as that. #Although, the label might fit me, or anyone else who has ever tested a two way radio. #http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
07-04-2007, 06:00 PM
this is one of the devices i have, its a pace 5403a swr/watt meter
07-04-2007, 08:16 PM
That is a "CB" SWR meter; it is designed for the 27 MHz band and is unlikely to be of use at 2-metres/144 MHz.
It's possible that it's equally-poor in both the forward and reverse power readings and that, since SWR is related to the ratio of those, it will give a reasonably-accurate reading of SWR.
I don't care to assume that; you really need an instrument that's suitable for 2-metres.
The design I gave you is about as simple as it gets to build; the alternative might be a good brand of VHF/UHF SWR/power meter.
"Two-way-radio tester" and similar terms were applied to "combination" instruments which could have several functions; signal generator, SWR meter, deviation meter etc.
The quality of these varied widely; it sounds as if you have one of the "CB" variety which won't assist you at 2-metres.
Now, if it were something like a HP 8960, things might be different. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
07-05-2007, 12:54 AM
07-06-2007, 01:14 PM
TV rabbit ears make 2 meter antennas that are simple and cheap. #
You will make a half wave center fed dipole. #Extend each of the elements out to a quarter wavelength of 146 MHz. #Two quarter wave lengths makes the half wavelength. 146 MHz is in the center of the 2 meter band and a good freq to size the antenna for. #It should work find on all the 2 meter FM frequencies.
Here is how to calculate that:
234 divided by megaherts = feet. #
234 / 146 = 1.6027 feet, which is 19.2324 inches, whic is so close to 19.25 inches that it won't matter.
Now, replace the 300 ohm twin lead wire with 50 ohm coax. #That is because this antenna has an input impedance of very close to 50 ohms, and quite far from 300 ohms. #Connect the center lead of the coax to one of the elements, right where you took off one of the twin lead wires. #Then connect the shield of the coax to the other element, right where you took off the other wire of the twin lead. #It won't matter which on is which.
When you extend the elements, extend the largest section out first, then the next smaller sections, until you reach the 19 1/4 inch length. #Do both the same.
If you want to make sure the elements don't accidently get longer of shorter, you can: # 1. #solder them in place, 2. #put a piece of electrical tape at each place they could change length #3. #Make a small file notch at each place and use that as a marker. #Also, the sections should "snap" into place at the filed notches.
I use this exact antenna as an emergency antenna for emergency communications on two meters.
REMEMBER: #when using this for 2 meter FM, it needs to be orientated in the veritcal position (up and down) but when working other modes it would be orientated horizontaly. #When mounting this antenna, keep the elements away from metal masts. #I use a 15 foot metal painter's telescoping extension for a mast with a 2 foot piece of wood doll on top of that so that this little dipole is well away from any metal objects.
Good luck and have fun. #Building your own antennas is a lot of fun.
07-07-2007, 02:01 AM
I built but have not used the ground plane mentioned by AI4OF and used a piece of wood to mount the So-239 and used the wood to mount the downward elements. I then connected the downward elements electrically. Any reason that this would not work? I was just playing in the garage and wanted to make it look a little more "refined".
07-08-2007, 09:33 AM
All these are great suggestions and antennas. If you have a local library close by, check it out to see if they have a copy of the ARRL Handbook.There is tons of info on some simple homebrew antennas for VHF. Welcome aboard and have fun!
07-08-2007, 06:38 PM
Quote[/b] (kb9mnm @ July 07 2007,21:33)]All these are great suggestions and antennas. If you have a local library close by, check it out to see if they have a copy of the ARRL Handbook.There is tons of info on some simple homebrew antennas for VHF. Welcome aboard and have fun!
i actually just found a copy of the handbook at the flea market. a nice thick useful book. 50 cents http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif
07-09-2007, 02:51 PM
NNR: The ground plane you built should work. Here is the theory and then reality.
IN THEORY, by not having the ground plane elements connected to a metal mounting plate, it would change the take off angle of the signal and also the input impedance of the radiator (which would change the SWR readings of the antenna).
IN REALITY, what you did by mounting on wood and electrically connecting the ground plane elements will probably have no noticable effect on the performance of the antenna. If any at all, you may get a less than perfect SWR, but still well within 1.5:1 (assuming the demensions of the elements are correct). By using wire to connect the ground plane elements ("radials") you may have actually lengthened them. If you have problems getting the swr below 1.5:1, you may try measuring the total length of the radials plus their connectors, from their electrical connection point to the mount, to their ends, and adjusting to fit the "design's" length.
Hope this helps.