2M Collinear Antenna
I am considering constructing a collinear antenna from "Practical Antenna Design 140-150 mhz" book. My question is if I were to have one dipole element facing the exact (180 degree) opposite direction would it work? Please refer to the website for an illustration: http://www.intellin.org/books/antenna_html/b-s22-1.htm
Additionally, the gain is 3 dB compared to a single dipole, so would this setup still yield that dB gain???
Any and all constructive input would be greatly appreciated.
Yes, you can do that if you're careful to:
-Make sure the elements are still spaced the correct distance apart, vertically.
-Make sure they're still in phase; in other words, build it exactly as shown then just rotate one of the dipoles around the mast so it faces the other way.
You'll get slightly less gain this way, but a more omnidirectional pattern. The 2-pole array shown in the article is not really omnidirectional, it produces a cardioid pattern favoring the side of the mast where the elements are.
By the way, the construction article contains a critical mistake and must have been authored by somebody who really didn't know anything about impedance matching!
I am a recently licensed ham and any advice would be helpful as to why. The reason I had chosen this antenna was because of its gain. Would you recommend any other antenna that I can build that has high gain and fit in the attic(thanks to hoa restrictions). I was looking to purchase the comet GP-3, but did not want to spend that much.
Originally Posted by WB2WIK
High gain arrays for VHF - first I gotta ask, 'Why?' IF you've got a reason, cool. Doesn't even have to be what I'd call a 'good reason' - just go at it intentionally.
VHF gain antennas are often used for SSB (horizontal polarization), satellite work (circular polarization) or other 'non-FM/repeater' type ops. They're also often used on repeaters themselves. It's a good idea to put the gain there so lower gain mobile stations can still 'get in' to it. (look up 'link budget'). Can also allow for lower power out from the repeater's radios and still get the desired coverage - this can also lead to extending the life of the repeater.
For most VHF type installs a 1/4 wave ground plane or J-pole will do the job quite nicely. Another good antenna is a 5/8ths wave base load. You'll see that often on mobile setups. Point is, before spending piles of effort for a complicated folded dipole array - evaluate the need for such a setup. Higher gain may just bring in more noise - not more communication.
If you've decided to go for a gain array - You may want to look at something like a Decibel products 222 or 224 for examples - these are dipole arrays often used in commercial installs. Look at some of the existing installs - likely a few around town - you'll see many with opposing bays (180 apart). You'll also see 4 bay arrays with each bay at 90 degrees.
Another good antenna is a 'hair-pin colliniar'. MUCH easier to build and install. Can even be 'hidden' inside a plastic flag pole. Check this page - several of these and many others to choose from as well!
And that's just one of literally hundreds of project pages out there.
And of course - for a hidden 'attic' install - don't forget the classic 300 ohm twin lead J-pole!! Popular first antenna of many hams and for a reason. They work!
::An educational experience might be had by purchasing the ARRL VHF Manual or RSGB VHF Handbook, both of which are absolutely excellent publications.
Originally Posted by KJ4EKS
Unfortunately, the chagrin of many, the best information on almost anything is not on the internet because authors wish to be paid for their writing, and their writing is copyrighted and not for general distribution -- and that's the sad but important fact of the internet.
The error in the construction plans of the article you referenced is that the writer used 50 Ohm coaxial cable to make the phasing harness. That isn't going to work, because two 50 Ohm antennas combined create a 25 Ohm load.
The phasing harness should use odd quarter wavelengths of 75 Ohm coax.
The author should have known that, but evidently didn't.
Thank you for your responses. KA7O you are right, maybe I shouldn't since I won't be doing any SSB/DX work.
My reasoning behind it was since I am surrounded by two story houses (mine is only one) and live in an h.o.a. commando haven, a good solution (to be able to reach repeaters effectively 30 miles away) was to put up a good gain antenna in the attic. Maybe I should try the 300ohm twin lead j pole you mentioned???
PVC & Copper
It may be easier if you use PVC & copper with a hairpin balun. As long as your spacing & harness are precise, it should work well. Like this, but with a 5 inch hairpin balun...
Originally Posted by KJ4EKS
Last edited by W7LPN; 07-24-2008 at 05:03 AM.
I am not defending the author and have not even learned to walk yet in ham radio/ electronic principals, but would the reason behind the author using 50 ohm be because of the gamma match ----> http://www.intellin.org/books/antenna_html/b-d2f-1.htm
Hope it helps.
Well, I guess 3 dBd might be considered "High Gain" in some areas, however in VHF, it is hardly "High Gain".
Originally Posted by ka7o
One of the biggest disappointments I have heard from new Amateurs is VHF FM repeaters. Too often folks forget about simplex on FM. A "High Gain" antenna reallyh comes in handy for Simplex operations.
Things are pretty flat in the West Palm Beach, FL area, so three dB additional gain could well be worth the exercise, and would most certainly be a good antenna project.
If the dipoles are mounted onto a mast which is relatively transparent to RF, it can be omnidirectional with both dipoles on the same side; however what Steve (WIK) posted is true, on a reflective mast both antennas on the same side will give a cardioid (heart-shaped) pattern.
73 DE KAØGKT/7