2-Meter Ground Plane Antenna in Attic

[Northmountain]

From the Guardian at Survival Podcast Forum

I thought I would pass along some information on my 2 Meter base antennas for all here. I was extremely pleased with the results of my recent antenna "upgrade" and felt I needed to share.

I started out my 2 Meter base rig with a simple horizontal, 1/2 wave dipole antenna for several reasons. It is an extremely economical design and easy to build. I actually purchased my first antenna, a 1/2 wave dipole, since I didn't know much, if anything, about building my own antenna at the time. Also, since I have restrictions in my subdivision, I had to install the antenna in my attic and the dipole takes up very little room. Once it was installed, I could hit local repeaters without too much issue but noticed some deficiences in coverage and somewhat lacking longer range performance. So, I did a little research and found that the horizontal dipole is not a very efficient design for 2 Meter.

During my "dipole" research, I found a number of recommendations for a simple ground plane antenna (single vertical 1/4 wave radiator with four ground radials, at 45 degrees to the ground and 90 degrees from each other). I also found that these antennas were a piece of cake to build. It seems that if you can solder, you can build one of these 1/4 wave, ground plane antennas. So, I set out to build one of these antennas but needed a few parts first.

What I needed:
SO-239 Chassis Mount Connector
Conductors - I used the ground wire from a section of 14-2 W/Grnd Romex
Soldering Iron & Solder
Non-Conductive Support - I used 550 cord for this

The only item from the list above that I had to purchase was a $3 SO-239 Chassis Mount Connector.

SO-239:


Below is the set of instructions that I used for the build:

http://www.hamuniverse.com/2metergp.html

After a little soldering, some rosin splattered on the kitchen table and scraps of romex on the linoleum, I had one ground plane antenna. A few minutes later, I was up in the attic replacing the dipole with this unit.




The antenna is actually suspended with 550 cord through a loop that I put into the end of the radiator/vertical mast. Remember, your overall radiator length is from the insulator of the SO-239 up to the end of the loop if utilized.

The antenna was held vertical by a horizontal piece of cord wrapped around the SO-239.


Also, take note that although this installation is mainly protected from the weather in the attic, it can get very humid up there. So, I used some liquid electrical tape to seal the coax and connector interface to prevent water from getting into the coax. Water intrusion is the number one enemy of coax.

OK, so the antenna is installed, but would the performance of the antenna be any better than the dipole? A short answer to this question... YES, YES, YES! I was able to hit repeaters up to 60+ miles away this evening! The 1/2 wave dipole didn't even come close to this! Reception has also increased!

While tuning the antenna, I just made one trip to the attic and called it good. I started with an SWR of ~1.5:1 at 144 MHz and got down to ~1:1 at 145.075 MHz. Wow, I was really impressed! At 148 MHz, I found that the antenna had an SWR of ~1.3:1. The highest SWR I had was 1.5:1. With those results from a single tuning, I figured that I would call it good. While I could bring the 1:1 closer to my op frequencies right around 146 MHz, I felt that what I got was just fine for now since I also work some repeaters down around 145 MHz. Besides, I had very little time to get this project completed and was lucky to even get it done!







EDIT: I did some further SWR testing and found that the SWR was better than I had found last night. So, I updated the post as well as the SWR chart for the antenna


MEASUREMENTS FOR 146 mhz

The vertical element and radials can be made of #12 copper wire or welding rods, coat hanger, etc. The vertical radiator (A) should be soldered to the center connector of the SO239.

The four base radials (B & C) and (D & E) can be soldered or bolted to the SO239 mounting holes using 4-40 hardware. The four base radials then should be bent downward to a 45 degree angle.

The antenna can be mounted by clamping the PL259 to a mast or even passing the coax through a 3/4 ID PVC pipe and compression clamping the PL259. Either way let your creativity work for you. If you plan on mounting it outside, apply RTV or sealant around the center pin and PL259, and TAPE WELL, to keep water out of the coax.

Make each radial a 1/4 wave of your desired xmit frequency. Sometimes it helps to add a little extra length to the radials and radiator. This will give you some adjusting room when you adjust the SWR.
(If adjustment is needed, clip all radials equally about 1/8 inch at a time while checking SWR, USING LOW POWER). Center the lowest swr on your transmit operating frequency.




Example Calculation:

Freq (mhz) 146
A (inches) 19 5/16 (Note "A" length is to the SO-239 insulator but not critical)

B THRU E (INCHES) 20 3/16

LENGTHS FROM FORMULA ( 234/FREQ MHZ) + 5 % LONGER FOR RADIALS

TRY ONE ON 440 or other bands USING SAME FORMULA AND CONSTRUCTION TECHNIQUES!

See online antenna radial and radical length calculator here.

[AZ Prepper]

This is great! Thanks for sharing!

[Northmountain]

Just added link for an online calculator for antenna radical and radials. Build your own ham antennas fast and easy. In my experience, the longest part of the construction of this type 2-meter antenna is stripping the #12 wire in the romex bundle.

[Kelly Alwood]

very nice...im gonna try it...thanx for the post...

[Northmountain]

It works great Kelly. Check it with an SWR. If you've cut it exactly to length per the instructions, it should be really close.

very nice...im gonna try it...thanx for the post...

[Kelly Alwood]

where will i adjust the swr? at the connection to the so239?

[InDy]

Very cool, NM! Thank you for sharing.

[Northmountain]

Sorry to be slow to respond Kelly. Measure it at the radio end. I continue to be amazed at the range and reception I'm getting from this little jewel. Read the article for the 'sweet spot' SWR / Frequency ratio .... assumes you have your radials trimmed exactly the same lengths as in the article.