Tuning a 5/8 wave antenna for FM broadcast

[carlvenorden]

Is there a better way to determine the length of the verticle radiator (the part that slides up and down) for 97.7fm?

Is using a SWR meter the "best" way to determine how well you are tuned?
Thanks so much,
Carl

 

[rbrucecarter5]

Is using a SWR meter the "best" way to determine how well you are tuned?

What you are doing with the SWR meter is making the best impedance match possible with your transmission line and transmitter output. You did not mention your output power, but having a mismatch is not only a potential coverage problem, but reflections can also stress your transmitter - causing early failure.

 

[BobOnTheJob]

Most transmitters have an "SWR" meter included. Set the meter to Reverse Power or Reflected Power. Adjust the transmitter to the lowest power that will give you an indication on the above mentioned metering position. As you get the reading lower from adjustment, the power can be increased to fine tune the antenna further. I'm assuming this is for an LPFM station. Of course, the transmitter needs to be turned off while making antenna adjustments...for the adjuster's safety as well as the transmitter's well being. In the event that the transmitter has no SWR meter within, a VHF SWR meter such as a Bird model 43 with a slug for the correct frequency range and power will work equally well. Do NOT use a CB style SWR meter as these will NOT provide accurate indications at FM broadcast frequencies.

 

[Neil E.]

Carl,

Since I know you from other posts I am going to make a couple of assumptions for which, if I err, please forgive me.

If this is a part 15 application it may be difficult to get a meaningful reading from a SWR meter due to the low power typically involved. SWR readings are very useful on the 146 MHz amateur bands for antenna matching even though they are not "Bird" meters. Even the cheap meters which use directional strip lines work well for amateur radio work, though they may not be suited for commercial high power stations.

A 5/8 whip antenna requires more than just getting the length correct. Keep in mind that what you are trying to accomplish is to match the antenna to the transmission line. This requires a tapped coil (a voltage step up transformer) which is installed in the base support housing for commercial mobile VHF whips. Once the proper coil is installed, the minimum SWR is set by the whip length.

I know of no 5/8 wave whips available for the broadcast FM band, so if you are going to do this you will need to wind your own coil and calculate the whip length. The gain advantage over a simple 1/4 wave whip is not going to be worth the effort unless you just want to experiment.

Neil

 

[4dude]

Well a 5/8 wave is the best animal to try it with i think. (I know that its the best for 10,11,12 and 15 meters)

 

[Doctor_Technical]

I know of no 5/8 wave whips available for the broadcast FM band

Google "Comet 5/8 wave". Nothing fancy, but I know of several stations that it has kept on with at least some ERP. "Some" being better than "none", it's not bad to have one stashed away for emergencies.

 

[Ted Russell]

Yes, the Comet 5/8 wave antenna is easy to set up, inexpensive (as far as broadcast antennas go) and it is indeed FCC type accepted. I believe it can handle up to 200 watts, so it should be just fine for any LPFM. The only minus is that it is vertically polarized, so if you are looking to have vertical + horizonatal or circular polarization you will need a different antenna. There is a large school of thought that goes against the norm and claims that all you need is vertical. In the case of LPFM it probably doesn't matter. The antenna has a gain factor of 3.5 db. There are LPFMs using this as their primary (only) antenna for several years now.

 

[carlvenorden]

I'd like to thank Neil and everyone for your suggestions and opinions.
Actually the antenna is, if not by name, the same as the Comet 5/8 wave.

This is a VERY VERY low power transmitter, and less than 1/10th of a watt.

We are attempting to provide coverage in an area that is not only hilly, but has deep valleys, and with our previous 1/4 wave antenna, more or less a starduster ground plane, we had good coverage on the hills but completely fizzled out in the valleys. What makes this worse is that we have only a few local roads, all of which are curvy and contain constant hills and valleys. We are directly behind a HUGE development of houses, shrouded by a small woody area. So, potentially this very small signal can give us a large listenership.

I should point out that we feel, if we were living on flat ground that we would have a nice signal over this development, and not much else territory, but because of our terrain, we have a terrible time covering much of it.

These FM-type antennas arrive with very vague directions as to tuning. FYI, the instructions for both the "comet" and the 1/4 wave came with instructions that gave a one inch measurement for over 10 channels; which means one size fits all. We know that is not the case. Fine tuning the antenna is our only hope for not only getting out a decent sounding signal that is more flawless, but we also know we are not going to have a heck of a lot of range, and we didn't plan to have a lot of range anyway.
Both antennas arrived with measurements in millimeters, which is fine and good if you are familiar with metric measurement, however we are not. Therefore I did a rough translation and came up with what I thought was a proper length.

In converting from the 1/4 wave antenna, which has a tendency to shoot signal to the ground (which I thought would be a good thing, because we are higher up than most of the surrounding land), to the 5/8 wave with a 3.5 dB gain, we felt that we would fill in the holes in the signal in the low lying areas. Our goal was not to attain a further range per se, but to 'just' fill in holes. We also felt a more verticle signal would help cover the landscape, and this does not appear to be the case. Of course we are on a budget and we are not a commercial installation, just a service to the community. So our equipment is more or less consumer quality....all around.

Points made about the swr meter were good ones, especially since we have had to move the studio and the audio line is twice the length it was. The transmission line, very simple coax is probably not more than a foot long, since the tx is mounted part 15 style just beneath the antenna.

The difference in the signal is what prompted me to ask these questions.

We do intend to try a SWR meter between the antenna and tx, to see if we have a high reading; if we do, we will adjust the antenna length to lower the swr. I am hoping that in doing so, we will have tuned the antenna correctly and thus get the best signal out of the antenna that we can have. In the meantime I have recalculated the measurement of the antenna length and we can check it that way as well.

It has been suggested by an engineer, who is not a poster on this board that we should run a ground wire from the FM tx to the ground, using a length of copper pipe as long as the wavelength is long. And we think we will do that to see if it gives the signal a little more punch. The antenna is grounded to the building's electrical system for safety, though.

Are there any more comments to add to what I said above?
I thank one and all for your advice!

 

[Chuck]

Are there any more comments to add to what I said above?
I thank one and all for your advice!

Since you have directions for tuning this antenna in the Metric system, how about going to Wal-Mart and buying a tape measure that is both English and Metric? They are very common and don't cost much. If your transmitter power is 1/10 of a watt, you will have a hard time finding an SWR meter that can read that.

It is also fair to point out that 1/10 watt into an antenna with 3.5 db of gain is not likely to be Part 15 compliant, even if you don’t have a perfect match to the antenna. Of course, I have no idea where you are, nor do I want to know, so it may not be an issue in your neck of the woods.

 

[Chuck]

One other thing that can really affect your performance is the feed line between the transmitter and antenna. You can find charts on the Internet that give you exact figures for various cables, but it isn't that hard to have a 3-4 db (or even more) loss in 100 feet of cable at 100 MHz. That's why commercial installations always use very large and expensive rigid or foam filled feed lines. Some hobby broadcasters try using cable like RG-58 from Radio Shack. Worse yet, some even try using 75 ohm TV antenna cable with their 50 ohm transmitter and antenna. That causes a mismatch and even more loss. All these common cables have huge loses at 100 MHz. By the end of the run there is little signal left.

The best idea is to put the transmitter as close to the antenna as you can get it (like next to it on the mast) and use a short jumper made of the lowest loss stuff you can find. RG -8 is OK for a few feet, but better would be Belden 8813 which, naturally, costs a lot more. Incidentally conventional "UHF" aka "PL-259" connectors also have a lot of loss at these frequencies. A better choice is an "N" connector. Of course, your antenna may not give you any choice. I think most Ham Band Comets use a PL-259. I suspect yours does to. But you don't have to duplicate the problem on the transmitter end. The less you can put between the transmitter and the antenna, the better.

 

[CW]

We are attempting to provide coverage in an area that is not only hilly, but has deep valleys, and with our previous 1/4 wave antenna, more or less a starduster ground plane, we had good coverage on the hills but completely fizzled out in the valleys. What makes this worse is that we have only a few local roads, all of which are curvy and contain constant hills and valleys. We are directly behind a HUGE development of houses, shrouded by a small woody area. So, potentially this very small signal can give us a large listenership.

I should point out that we feel, if we were living on flat ground that we would have a nice signal over this development, and not much else territory, but because of our terrain, we have a terrible time covering much of it.
<snip>
In converting from the 1/4 wave antenna, which has a tendency to shoot signal to the ground (which I thought would be a good thing, because we are higher up than most of the surrounding land), to the 5/8 wave with a 3.5 dB gain, we felt that we would fill in the holes in the signal in the low lying areas. Our goal was not to attain a further range per se, but to 'just' fill in holes. We also felt a more verticle signal would help cover the landscape, and this does not appear to be the case. Of course we are on a budget and we are not a commercial installation, just a service to the community. So our equipment is more or less consumer quality....all around.
<snip>
We do intend to try a SWR meter between the antenna and tx, to see if we have a high reading; if we do, we will adjust the antenna length to lower the swr. I am hoping that in doing so, we will have tuned the antenna correctly and thus get the best signal out of the antenna that we can have. In the meantime I have recalculated the measurement of the antenna length and we can check it that way as well.

It has been suggested by an engineer, who is not a poster on this board that we should run a ground wire from the FM tx to the ground, using a length of copper pipe as long as the wavelength is long. And we think we will do that to see if it gives the signal a little more punch. The antenna is grounded to the building's electrical system for safety, though.

Are there any more comments to add to what I said above?
I thank one and all for your advice!

1) A 1/4 lamba antenna is better for hills and flat area combined....a 5/8wave will put MORE signal into the lower angles of radiation....thus NOT covering the valleys, etc (think a donut as the 1/4wave....then squeeze it flatter and now you have a 5/8wave...but the signal covering the valleys is NOT there anymore as it has been SQUEEZED into the tighter pattern looking at the horizon!
2) A 5/8wave is NOT tuned by trimming it...for best take off angle, a real 5/8wave will only be resonant at ONE freq...thus it needs to be cut to the correct length and then MATCHED with external components to 50ohm match...
CUTTING the antenna WILL affect the electrical length....thus affecting the radiation pattern...true, it will be close BUT if you want to be picky about it, you dont want to trim the antenna for best SWR....the matching network is made for that...the antenna needs to be the PROPER length and left alone (like an AM vertical)
3) A 1/4 wave does NOT radiate "Into the ground" as noted above....that is a false and incorrect statement....for your operation, I would suggest running CP...try making one with two 1/2wave dipoles at 45deg to vertical (and 90 deg to each other) and fed with a 90deg phasing section of 70ohm coax... would look like an X straight on....you could stack two or more for better gain...a 5/8wave in my opinion will not be worth it on a FM channel vs a 1/4wave...
4) A ground wire will NOT give you any advantage here in radiation...it MAY help in lightning protection but thats it...Your antenna (I take it the current 1/4wave does have a ground plane on it?) is where you radiate...NOT the ground wire from your transmitter!

 

[rfry]

Actually the antenna is, if not by name, the same as the Comet 5/8 wave.

This is a VERY VERY low power transmitter, and less than 1/10th of a watt.

We are attempting to provide coverage in an area that is not only hilly, but has deep valleys, and with our previous 1/4 wave antenna, more or less a starduster ground plane, we had good coverage on the hills but completely fizzled out in the valleys. ... I should point out that we feel, if we were living on flat ground that we would have a nice signal over this development, and not much else territory, but because of our terrain, we have a terrible time covering much of it.

Good FM coverage depends far more on the existence of clear, line-of-sight paths between the transmit and receive antennas than on transmitter power and/or the choice of a 1/4-wave vs a 5/8-wave antenna.

For equal applied power, a 5/8-wave vertical radiates more field strength within +/- 25 degrees of the horizontal plane than does a 1/4-wave vertical. So depending on the terrain there, coverage into the valleys possibly could improve (marginally) with the 5/8-wave.

The length of a 5/8-wave antenna should not be trimmed to improve the match, because the best match will call for trimming it to considerably less than 5/8-wave -- and of course the antenna won't have the gain and pattern of a 5/8-wave radiator, then. The input Z of the antenna at its appropriate 5/8-wave length needs to be matched to the impedance of the transmission line connected to it (probably 50 or 75 ohms).

Lastly, Part 15 allows 1/10 of a watt of DC input power to the final r-f stage of the transmitter for the AM carrier, but not for FM. The FM rules state that the maximum field strength in any direction 3 meters from the transmit antenna cannot exceed 250 µV/m. Part 15 FM transmitter power is undefined. So if a "high gain" antenna is used, the output power of the transmitter needs to be reduced so that the peak radiated field does not exceed that limit.

It takes very little power radiated by even a simple 1/2-wave dipole (gain = 2.15 dBi) to generate the Part 15 FM field limit -- about 11.4 nanowatts (0.0000000114 watts).

R. Fry http://rfry.org

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