1/4 Wave Verticle Antenna for the FM-02

[JHinkle]

Great discussion!! I do have a couple of questions though.

(1) Why would the GP antenna not be susceptible to CMC on the feedline? I did a google search this morning and read a couple of articles that were written about CMC on the feedline of a GP antenna.

(2) Any issues with adding some 45 degree fittings on either side of the T and bending the radials 45 degrees to get the feedpoint impedance closer to 50 ohms?? Do you not want the radial to come into contact with the PVC???

edit: Here's one of the articles I pulled up and read this morning : http://www.w8ji.com/ground_plane_verticals.htm

Question #1. The Vertical Group Plane antenna is not a balanced fed antenna. It has one active element and the ground plane so it matches that of the coax.

Question 2: You can but no need. Yes you will have an impedance mismatch and SWR will not be 1:1 - but if you reach your listening area - why do it. If you were building an antenna to transmit as far as possible - that would be a different story.

Joe

[IdunBenhad]

Hi:
Ham radio operators have successfully operated and used both ground planes and dipoles and many other types of antennas for years before "CMC" was even a gleam in someones eye.

When dipoles and ground plane antennas are discussed, there is not a great deal of difference in the two antennas. If a dipole is hung vertically and the ground side of the antenna is extended horizontally, guess what? we have a crude ground plane.

A dipole and/or a groundplane are not usually considered balanced feed antennas, the distinction being that the coaxial cable feeding them has a grounded shield and a center conductor. If the reader is interested in "balanced feed" antennas, look up a folded dipole. That is "balanced feed". There are many such antennas in use by hams, but most of these require an instrument called an "antenna tuner" to match the unbalanced output of most common transmitters to the balanced feedline.

Actually, "antenna tuner" is a misnomer, as the device does not tune the antenna at all, but is merely an impedance matcher between the transmitter and feedline/antenna. There is nothing that can be done at the transmitter end of the system to change the antenna itself.

However, we go far afield in the above discussion. As both Joe and I have tried to get across is that the "proof of the pudding" is in the operation and range and how well you cover your display with good signal strength and clarity of signal.

[mikentn]

Thanks for the replies, gentlemen. I was thinking from reading Joe's paper that the common mode currents were negated by using an unbalanced antenna design, until I read the article I attached and some others I found. That's why I was asking for the clarification. My concern isn't so much for the transmission area as it is the protection of the transmitter. I remember reading some posts last year about some forum members' transmitters giving up the ghost, and I believe that's when RF Burns brought up the idea of a low power VSWR and a low pass filter for the output of the transmitter.

edit: Sad to say, but I actually took an antenna theory class many moons ago while in college. What I remember from that class I could fit into a ferrite bead casing.

[JHinkle]

Thanks for the replies, gentlemen. I was thinking from reading Joe's paper that the common mode currents were negated by using an unbalanced antenna design, until I read the article I attached and some others I found. That's why I was asking for the clarification. My concern isn't so much for the transmission area as it is the protection of the transmitter. I remember reading some posts last year about some forum members' transmitters giving up the ghost, and I believe that's when RF Burns brought up the idea of a low power VSWR and a low pass filter for the output of the transmitter.

edit: Sad to say, but I actually took an antenna theory class many moons ago while in college. What I remember from that class I could fit into a ferrite bead casing.

I know what you mean - I took a graduate level class on Antenna theory - yah - I also had a design class on vacuum tubes.

I read the article - can't say much except about the statements in the article except they appear to be made based on the simulation tool he is using. I did a google on the tool and could not find it. I have not found any formal text books (even the ARRL books) that talk about common mode current when addressing ground plane antennas - only balanced antennas like dipoles, yagis, etc.

Here is the bottom line - I wrote the paper to shed light on why someone might be having an issue. I believe if your signal is being received, with clarity, in the area of your interest - you won't have problems that can destroy the output transistor in your FM-02. If you can be heard in front of the house - be happy - we are not attempting to build a perfectly tuned systems with every bit of transmission distance squeezed out. If you are NOT getting your signal to your viewing area (in front of the house or two houses each side), then look at some of the possible reasons and see which one may solve it. Your installed dipole (even with common mode currents) is fine if your reception area is reached.

Select an antenna, define your listening area, build it, test your listening area (don't drive 3 blocks away and proclaim you have an issue) - if all is fine your done! If you are having an issue with reception in your listening area, Idun's SWR meter and understanding your system (transmitter, coax, antenna, position, height, orientation) - can help you identify the issue and address it.

Joe

[smeighan]

I know what you mean - I took a graduate level class on Antenna theory - yah - I also had a design class on vacuum tubes.

I read the article - can't say much except about the statements in the article except they appear to be made based on the simulation tool he is using. I did a google on the tool and could not find it. I have not found any formal text books (even the ARRL books) that talk about common mode current when addressing ground plane antennas - only balanced antennas like dipoles, yagis, etc.

Here is the bottom line - I wrote the paper to shed light on why someone might be having an issue. I believe if your signal is being received, with clarity, in the area of your interest - you won't have problems that can destroy the output transistor in your FM-02. If you can be heard in front of the house - be happy - we are not attempting to build a perfectly tuned systems with every bit of transmission distance squeezed out. If you are NOT getting your signal to your viewing area (in front of the house or two houses each side), then look at some of the possible reasons and see which one may solve it. Your installed dipole (even with common mode currents) is fine if your reception area is reached.

Select an antenna, define your listening area, build it, test your listening area (don't drive 3 blocks away and proclaim you have an issue) - if all is fine your done! If you are having an issue with reception in your listening area, Idun's SWR meter and understanding your system (transmitter, coax, antenna, position, height, orientation) - can help you identify the issue and address it.

Joe

All of this discussion about antenna types and impedance patching brought up an incident when i was in college, 1972. I belonged to a club of radio amateurs. One of them drove a 60p nail into a tree in their front yard and impedance matched it to their 10 or 15m transmitter. The tree became their antenna. It worked, i dont think it went that far.

Maybe i can use my tree in the front yard as my Antenna?

*smile*

[IdunBenhad]

Hi:
smeighan: Probably what was radiating was the feedline to the nail. Trees are a pretty good insulator/absorber of RF. And, you don't need much power to operate 10/15 meters if the band is open. 5 watts will work the world if the propagation is right.

Just thought I'd throw that in.

[ags0000]

I am one of the unfortunates from last season that had an FM02 die, and I used a simple dipole antenna. My FM02 may have been DOA, or I may have damaged it when putting it into the enclosure, or the antenna may have played a role; I doubt I'll ever know for certain. I had (and still have) a 1/2 λ dipole hanging in the rafters horizontally (it works fine with the EDM unit that replaced the FM02). I'd like to attempt a different antenna (just for kicks). Any suggestions on a simple antenna for low-power, preferrably with a 180 degree forward radiating pattern?

[IdunBenhad]

Hi:
ags000:I think if you read the opening entry on this thread and the white paper, you will have an answer. Joes' antenna is very easy to build.

One comment on the FM02 and the EDM. If the EDM is operating satisfactorily on the dipole and is not exhibiting any problems, then my conclusion would be that you had a bad FM02. Your setup and mine sound about the same and mine has worked perfectly for two seasons. I did have and reported one problem, but that turned out to be power supply noise that drove the FM02 crazy.

[ags0000]

Hi:
ags000:I think if you read the opening entry on this thread and the white paper, you will have an answer. Joes' antenna is very easy to build.

One comment on the FM02 and the EDM. If the EDM is operating satisfactorily on the dipole and is not exhibiting any problems, then my conclusion would be that you had a bad FM02. Your setup and mine sound about the same and mine has worked perfectly for two seasons. I did have and reported one problem, but that turned out to be power supply noise that drove the FM02 crazy.

Idun: Yes, Joe's paper is very well done, and I did read it thoroughly (and this thread as well). I appreciate the effort. My question about a different antenna design was specifically for one with a 180 degree radiating pattern. I am familiar with "normal" ground plane antenna design (using 3 or 4 horizontal elements) and believe they are omnidirectional. I've assumed that the 2-ground-plane-element design Joe has offered presents a similar radiation pattern as a "normal" design. I am also familiar with directional (beam) antennas, with much narrower radiation patterns. I'm wondering if a design exists that is a) simple enough for DIY construction and application; b) does not require precise tuning or equipment for matching; c) is small enough to fit in a low-pitch attic; and d) has a forward-biased (180 degree) pattern. My dipole is fine for my show purposes; this is driven more out of interest and curiosity (and optimization) rather than need.

Also, I do suspect that the FM02 was DOA, but without proof I am reluctant to make such a statement. My goal is to be fair and not cause concern in others unnecessarily. (ukewarrior was kind enough to send me a replacement final output transistor, which I haven't yet tried to see if it revives the FM02).

Edit: I'm looking for an antenna that will radiate 180 degrees around the vertical element (but simpler than a beam design).

[IdunBenhad]

Hi:
Without resorting to reflecting planes or additional elements, a 180 degree pattern is not likely. Once one gets into the realm of directional antennas, then design and construction becomes more difficult. A 180 degree pattern gets practical only in the UHF region because construction becomes easier to handle but dimensions become more critical.

Electronic equipment and parts are electronic equipment and parts. Having made that stupid statement, what I am getting at is that these parts can and will fail at any given moment even though the failure rate is now measured in years instead of hours. Only had there been a high rate of failure with the FM02 would there be cause for concern and telling about your experience should be counted as informative, not complaints.

The FM02 has a pretty good track record as far as I know.