Tell me about 1/4 waves

Santiam · March 31

I picked this little antenna the other day for my RT97 for back country communications...Easy to pack and easy to hide were my main considerations.

After playing with it for awhile I must say I am thoroughly impressed. Definitely out performs my roll up N9TAX that was my previous back country antenna.. I was able to talk to my repeater at 16 miles with a HT, it was high on a hill though.. I just had no idea this little antenna could be so good...

BoxCar · March 31

The radials coming off the base of the antenna makes it a half wave antenna.

Santiam · March 31

Now I am confused, I thought the radials were a ground plane.. Interesting..

BoxCar · March 31

They are. A radial adds to the length of the antenna making a 1/4 wave into a center fed dipole with a total length of 1/2 wave.

WRQC527 · March 31

19 hours ago, Santiam said:

I picked this little antenna the other day for my RT97 for back country communications...Easy to pack and easy to hide were my main considerations.

After playing with it for awhile I must say I am thoroughly impressed. Definitely out performs my roll up N9TAX that was my previous back country antenna.. I was able to talk to my repeater at 16 miles with a HT, it was high on a hill though.. I just had no idea this little antenna could be so good...

Yes, quarter-wave ground plane antennas work really well. I've built and bought several of them for 2-meter amateur radio use. The ones like yours that I've seen on Ebay have stiff straight radials bent down 45°. Are the ones you have drooping because they're flexible, or are they bent in a curve like that?

Santiam · March 31

39 minutes ago, WRQC527 said:

Yes, quarter-wave ground plane antennas work really well. I've built and bought several of them for 2-meter amateur radio use. The ones like yours that I've seen on Ebay have stiff straight radials bent down 45°. Are the ones you have drooping because they're flexible, or are they bent in a curve like that?

That is the Ebay one.. They are bent that way.. Had a note with it that said he has changed the radial design..Said this was better.. Told him I needed it tuned for 462.6500 and it came in 1.0 on channel 19...I didn't test any other channels as 19 is all I am going to use it for.. It seems to be a great little antenna..

Lscott · April 1

20 hours ago, Santiam said:

Now I am confused, I thought the radials were a ground plane.. Interesting..

They are. It depends are what point of view you take. The minimum length for a radiating element used as an antenna is 1/2 wavelength. Obviously a 1/4 wave alone won't work. That's where the ground plane come into play. The easiest way to understand how it functions is place your finger at a right angle against a mirror. You see a reflection of your finger so it looks twice as long. The same thing happens with a ground plane. It "reflects" the 1/4wave element making it look like a 1/2 wavelength long.

By bending the radials it changes the feed point impedance so it's closer to 50 ohms, a good match to the 50 ohm coax cable. A true 1/2 wavelength dipole antenna is closer to 70 to 75 ohms with the feed point at the center. That results in an SWR of about 1.5:1, which is perfectly acceptable by just about any radio out there.

A 1/2 wavelength antenna is used where a ground plane, or room for radials at the base of a 1/4 wavelength antenna, is not possible. The draw back is the antenna is now twice as tall and the bandwidth is reduced since a special matching section is required at the base of the antenna.

Also high gain antennas are multiple sections of 1/4 and 1/2 wave elements with a "phasing" section in between the sections. Those are the small bulges or tiny single loops you see spaced along the length. The down side to high gain antennas is the gain is HIGHLY concentrated in a direction perpendicular to the antenna. Works great for relatively flat open terrain. Not so good in hills or mountainous areas. People who like to hit the trails often carry two antennas. A high gain one, these antennas can be rather tall, used when on the highway and a low gain, usually a 1/4 wavelength, when hitting the trails.

https://forums.mygmrs.com/gallery/image/259-ca-2x4mb-jeepjpg/?context=new

https://forums.mygmrs.com/gallery/image/268-ca-2x4sr/?context=new

https://forums.mygmrs.com/gallery/image/338-cheap-14-wave-gmrs-antenna/?context=new

Santiam · April 1

WOW! Thanks for the in-depth explanation.. So cool..

Borage257 · April 2

Cut it for MURS (153 MHz) and it will be pretty good on GMRS too!

Lscott · April 2

31 minutes ago, Borage257 said:

Cut it for MURS (153 MHz) and it will be pretty good on GMRS too!

If he does that it will may look OK from an SWR standpoint but the pattern will look bad with multiple lobes likely giving crummy coverage. I wouldn't recommend it other than as an experiment.

Borage257 · April 3

@Lscott could you post a model of the Murs quarter wave ground plane on GMRS frequencies? I'm curious.

Lscott · April 3

OK.

I simulated a simple magnet mount 1/4 wave MURS antenna. The frequency was selected to be about in the middle of the band. The results looks good. The match and radiation pattern look like what I would expect.

The exact same antenna was then run at a frequency about in the middle of the GMRS band, between the simplex and repeater input frequencies.

As you can see the match short of looks OK on GMRS, however most of the RF power shoots up at a very high angle uselessly into space. I guess if GMRS has satellites in low earth orbit it might be a good antenna. For normal ground work most of the power is wasted.

This is what I was getting at in my prior post comments.

Lscott · April 3

If anyone is interested in messing around with antenna simulations the software is now freely available.

https://www.eznec.com/

I've attached some models I've played around with in the past.

Magnet Mount - Rev 2.EZ Magnet Mount Ham and MURS.EZ Ham and GMRS Magnet Mount.EZ GMRS J-Pole Rev 3.EZ GMRS 11 Element Yagi Rev 2.EZ FRS-GMRS Gnd Plane With Radials.EZ 4 Bay Anetnna MURS.EZ

Lscott · April 3

Here are a few more models. These antennas are a bit less common. The worse one to build and model was the 11 turn helix. I was looking at that for a satellite UHF down link. In the end I went with an 11 element Yagi from M2 Antennas. It was smaller, far easier to aim and the gain was about the same. The only thing lost was the circular polarization.

https://antenna-theory.com/antennas/travelling/helix.php

https://www.m2inc.com/categories/commercial/antennas/helical.html

The rest are a bit more common.

Corner Reflector:

https://www.arrowantennas.com/solid/cr4501.html

Eggbeater:

http://146970.com/PDFs/Antenna Eggbeater-Revisited-English.pdf

Turnstile:

http://www.on5au.be/Cebik-2/SomeNotesOnTurnstileAntennaProperties.pdf

https://www.qsl.net/g4hbt/dipole.htm

The Turnstile model doesn't use the "typical" dipole phasing cable setup.

Turnstile.EZ Eggebeater Rev 8C.EZ 90 deg 70cm Corner Reflector Rev 2.EZ 11 Turn Helix.EZ