1/4 Wave Verticle Antenna for the FM-02

[JHinkle]

I received my FM-02 from Chris at WLC recently.

I searched the forum and found most were using a dipole antenna.

I found many threads where people were having issues with their dipole antenna and could not identify and implement a solution.

I provide the attached white paper to the forum which outlines many of the technical aspects of antenna system design so it may help those with problems, figure out and address their problem.

At the end of the paper I offer a simple 1/4 wave vertical antenna (construction is very similar to the dipole plans presented in the WIKI) which, by design, side steps many of the potential issues associated with driving a dipole antenna. The Feed-Point impedance is 50 ohms and the antenna can be directly connected to the coax feed without fear of "common mode currents" and the issues they can present.

Joe

http://www.joehinkle.com/DownLoad/Antenna Design for FM.pdf

[Festus]

This is a great paper..... how did you make the loops to attached to the eyebolts? I have used very similar techniques during my days in the Navy as a tactical communicator. Many times we were required to set up covert expediant antennas in urban environments with materials we found in the immdediate area....had to blend in with the "trash" in the area

[JHinkle]

This is a great paper..... how did you make the loops to attached to the eyebolts? I have used very similar techniques during my days in the Navy as a tactical communicator. Many times we were required to set up covert expediant antennas in urban environments with materials we found in the immdediate area....had to blend in with the "trash" in the area

I think they talk about it in the dipole construction article in the WIKI.

I stripped 1.5 inches of insulation off the end of the element and looped it back onto itself. Soldered the end to form a loop. Used a tie-wrap to looped between copper loop on element and eye bolt (insulate the antenna from the tension bolt).

Joe

[IdunBenhad]

Hi:
Great write-up and paper. A ground plane is a very effective antenna and if constructed properly, performs very well. I have used them for many, many years as my only antennas for ham radio.

If a dipole is constructed properly, there should be no performance difference in the two antennas near the display.

If you want to measure the SWR, see my paper in Radio Waves: "VLP SWR Meter". See also "FM02 Transmitter Setup" and "SWR Meters & Antennas". These might be of help and info to you.

[IdunBenhad]

Hi:
Joe, I applaud your efforts and the white paper, which shows a lot a research and "diving" into the books. Great job.

I do however, have a question.

Were you able to measure the SWR? I am curious as to what it might be given that you have only two radials and those are at right angles to the driven element. Theory says the GP antenna will have a feedpoint impedance of about 38 ohms with the radials at right angles. If the radials are bent down at a 45 degree angle, then the impedance gets closer to 50 ohms.

The difference between 50 and 75 ohms and the difference between 38 and 50 ohms in most cases is not really significant.

Your design is terrific and can be much more easily mounted in a lot of cases than a dipole, so I see a good use for both antennas, depending on the users situation.

The whole idea here, of course, is to provide the DIYCer with an uncomplicated and effective means of getting his signal out to the masses, and you have done so with your design.

I think I may give it a try myself when I get the time. In my particular case, I have no problems with mounting either antenna "in the clear", even though my dipole is hung in the wooden rafters of Command Central, better known is IdunBenhad Lab without the white coats.

Again, good job. I am not being critical, just asking a question.

[JHinkle]

The design is a book design. That's the EE background in me coming out - I'm use to complex math and Smith Charts - not the practical application from being a Ham (I'm not - wish I was).

Idun, you are right (I can't find the reference that stated my implementation was 50 ohms - Looking at ARRL Handbook page 21.28 - 50 ohms is with multiple 30 degree sloping radials and 30 ohms is with perpendicular radials).

I did not measure SWR. Personally, I don't think it's necessary. I know there is a mismatch - as long as my end goal is met - SWR to me not important. Besides, the only way to measure SWR is to build one of Idun's custom meters (great job).

I believe the "best" instrument for the DIY individual is an FM receiver (in your car). The DIY should define the limits of their transmission area (front of house, 3 houses either side, 1 block either side, etc) and see if their Christmas music is clearly received. If so - Done. The DIY is not trying to maximize and tune the antenna system - most are just interested in letting people hear their sequence in front of the house.

Here is why I wrote the paper and presented it to the forum: I believe the dipole antenna is the wrong design for the DIY community - too many land mines to step on in the execution of the system - here is why:

Two biggest land mines - orientation and "common mode currents".

1. The dipole antenna, as presented in the Wiki, defines the construction and orientation as being "vertical". Vertical is the correct orientation - the antenna presents a more stable feed point impedance when positioned vertically at various heights off the ground. I have read many threads where people have placed their dipole in a horizontal plane. Feed point impedance is all over the map when a dipole is orientated horizontally and at different heights above the ground plane. All of these feed point impedance swings is what can be measured using a SWR meter. Just moving a horizontally positioned dipole up a couple of feet may make the difference in reception at the edges of your defined reception area.

2. A dipole antenna positioned in a horizontal plane and oriented with the antenna's end point pointing in the direction of the reception area (street) is a land mine. The dipole's radiation pattern is greatest perpendicular to the dipole axis and weakest along its axis. So if the DIY mounts his dipole horizontally (they hang nicely on the wall) - end points towards the street - positioned at a height where feed point impedance is such that most power is reflected, not transmitted (hi SWR) - odds are their music may not be heard in front of the house.

3. I believe the biggest land mine in using a dipole as a DIY is "common mode currents (CMC)" that come about by directly connecting the coax to the dipole without using a balun at the feed point. The question is not whether you will have common mode currents or not (that's given) - the question is how large is the current and how is it going to affect the health of your FM-02 ans the reception area.

The size of the common mode current will depend on the length of wire from the antenna's feed point to RF ground (usually the copper stake driven in the ground by your house's electric meter). At the FM band frequencies (wave length around 9 feet) changing that RF CMC length by 2 to 4 feet can make a big difference in common mode impedance - hence the amount of power actually being radiated by the antenna. Example - plugging in your FM-02's power supply in a "different" wall socket in the house can make a big difference in your reception area (the common mode impedance changes as the RF CMC length swings between multiples of 1/4 and 1/2 wave lengths). Remember - we are dealing with a 30 mill-watt transmitter. Also, if the common mode currents are large, they can actually destroy the output transistor in your FM-02.

I believe common mode currents are the biggest reason many DIY'ers, using a dipole antenna, are not getting the reception area they want.

The paper offers three ways to address common mode currents and minimize their effects.

I believe the best antenna for the DIY is a vertical ground plane antenna. Big reason - usually not a bad match from coax to feed point impedance so SWR is low - but the big reason is no common mode currents.

The proper design of a vertical ground plane antenna is to use multiple radials and specific sloping angles - but the DIY is limited to plastic pipe as the supporting structure for the antenna - and - multiple radials makes it hard to hang from the rafters or from a wall without wires sticking out into space.

The proposed design uses 3 wires, one more wire (compared to the dipole), does not have a great ground plane (uses only 2 radials as opposed to many), does not have a perfect impedance match (SWR will not be 1:1), but is easy to build and has no common mode currents to worry about, and most DIY'ers will orientate it properly in the vertical position (hangs nicely on a wall). The actual SWR (related to actual radiated power verses reflected power) should be within accteptable limits where your FM-02 will easily transmit into your limited listening aea.

I purchased the $5 dummy antenna when I purchased my FM-02. By my design goal - THAT antenna was the only thing I really needed. Reception was clear everywhere within my defined reception area. What I did not like about it was that it kept falling over - very unstable when fully extended to it's 12 inch length (base is about the diameter of a quarter).

Again - my intent was to provide some insight as to why the dipole works for some but fails for others and also present an alternative DIY antenna design that works without the dipole land mines.

Joe

[IdunBenhad]

Hi:
Joe, you and I are in total agreement about the range needed and how to do it. I have long preached about confining the range to the immediate area of the display.

My simple dipole, as well as Dirknerkles' design on the WIKI seems to defy the theory about common modes, etc., but I cannot argue with the theory. It could be that I just happened to have good luck with the antenna, although I have tried it in various configurations with about the same good results no matter how it was oriented. Dirknerkle experienced the same results with his antenna.

I think what we must remember is that it is results that count. As long as the transmitter is not harmed by excessive SWR and the range is sufficient, then we got it made, no matter what antenna is in use. "I advocated this in SWR Meters & Antennas."

It is good that DIYCers now have a choice and from the various papers and how-tos can make up their own mind about how they want to do it.

Thanks for your reply. I found it to be very interesting and enlightening. Hopefully others will also.

[barnettd]

I used Dirk's design and havent had ANY issues with my transmitter, and my antenna IS oriented in horizontal mode. I am getting (guessing) around 200 or so feet from my transmitter (which is the legal limit) and havent had any complaints from any listeners watching the show that there is any distortions or noise on my rf signal...

Just my 2 cents here.

[mikentn]

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

[Jrd]

Very informational paper! Although you may want to proofread it again as there are some spelling errors (ie. 'cooper' where I assume you meant 'copper') and equations that don't add up. (ie. '1001.1' where you probably meant '101.1')

Also it would be even more informational if you mentioned Idun's VLP SWR meter at the bottom of page 3 where you are talking about SWR meters.