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What setup do you prefer for LPFM? Full Wave or Half Wave? And why?
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Neither. .75 or .8W spaced antennas have less downward radiation than either of the other two. Downward and upward radiation can bounce back and cause some degradation due to cancellation. If you have to choose one or the other of the first two, 1/2W is usually the better choice from a radiated signal perspective, however it does have less gain, per bay.
Seems like a case of "thinking too much" here. Common disease afflicting engineers.
1. It's a 100 watt station (or less, depending on HAAT).
2. FCC will arbitrarily reject any antenna scheme much below 30 feet above occupied areas (roof or ground), so downward radiation is not much a factor.
3. By the time you buy a 1/2 wave antenna, much less a multi-bay antenna, you could buy a bigger transmitter and just run one bay. Example: antenna 100' ag, 120' 1/2 inch foam feedline, erp 100 watts--single bay antenna requires 240 watts, 2 bay 120 watts tpo. Single bay SWR about $750, 2 bay, about $800 more. Put the $800 towards a bigger transmitter.
4. My experience, at these low powers, not much difference between single or two bay CP installation. However--couple of Class A's in town went to 1/2 wave antennas. Not impressed with coverage from new installation.
5. Building a tower? Using one bay keeps windload down, can put a single bay antenna on pole or at least above top guys (if guyed); harder to do with 2-bay, expensive to replace std. guys with fiberglass or ($$$!) Phillystrand.
1. It's a 100 watt station (or less, depending on HAAT).
2. FCC will arbitrarily reject any antenna scheme much below 30 feet above occupied areas (roof or ground), so downward radiation is not much a factor.
3. By the time you buy a 1/2 wave antenna, much less a multi-bay antenna, you could buy a bigger transmitter and just run one bay. Example: antenna 100' ag, 120' 1/2 inch foam feedline, erp 100 watts--single bay antenna requires 240 watts, 2 bay 120 watts tpo. Single bay SWR about $750, 2 bay, about $800 more. Put the $800 towards a bigger transmitter.
4. My experience, at these low powers, not much difference between single or two bay CP installation. However--couple of Class A's in town went to 1/2 wave antennas. Not impressed with coverage from new installation.
5. Building a tower? Using one bay keeps windload down, can put a single bay antenna on pole or at least above top guys (if guyed); harder to do with 2-bay, expensive to replace std. guys with fiberglass or ($$$!) Phillystrand.
A few comments...
3. By the time you buy a 1/2 wave antenna, much less a multi-bay antenna, you could buy a bigger transmitter and just run one bay. Example: antenna 100' ag, 120' 1/2 inch foam feedline, erp 100 watts--single bay antenna requires 240 watts, 2 bay 120 watts tpo. Single bay SWR about $750, 2 bay, about $800 more.
A "1/2-wave antenna" usually refers to the vertical spacing between the bays or elements of an array, in wavelengths. So by definition any 1/2-wave FM transmit antenna is a multi-bay antenna consisting of at least two bays.
The tx powers shown in the quote above are needed for c-pol antennas/arrays to produce 100 W ERP. Those powers are about twice as much as needed for 100 W ERP when using linearly polarized bays.
4. My experience, at these low powers, not much difference between single or two bay CP installation. However--couple of Class A's in town went to 1/2 wave antennas. Not impressed with coverage from new installation.
If the c-pol antennas had essentially the same r.m.s. gains per polarization, the same radiation patterns, and the same max ERP there would be no practical difference in the coverage they produce, whether they were full or 1/2-wave spaced. Any significant differences would be located on/near the earth within several hundred feet from the antenna site, normally.
Neither. .75 or .8W spaced antennas have less downward radiation than either of the other two. (i.e., full wave or half-wave spaced)
Actually a two-bay array using 1/2-wave vertical bay spacing has less downward radiation than the full wave spaced, and both a 0.75- and 0.8-wave spaced array (see link below).
These patterns are based on a c-pol element with 10% field at +/- 90 degrees elevation, however the gains are shown for linear polarization, and must be divided by two for the h-pol and v-pol gain of a c-pol array.
http://s20.postimg.org/jyxvv923x/LPFM_Pattern_Comparison.jpg
3. By the time you buy a 1/2 wave antenna, much less a multi-bay antenna, you could buy a bigger transmitter and just run one bay. Example: antenna 100' ag, 120' 1/2 inch foam feedline, erp 100 watts--single bay antenna requires 240 watts, 2 bay 120 watts tpo. Single bay SWR about $750, 2 bay, about $800 more.
A "1/2-wave antenna" usually refers to the vertical spacing between the bays or elements of an array, in wavelengths. So by definition any 1/2-wave FM transmit antenna is a multi-bay antenna consisting of at least two bays.
The tx powers shown in the quote above are needed for c-pol antennas/arrays to produce 100 W ERP. Those powers are about twice as much as needed for 100 W ERP when using linearly polarized bays.
4. My experience, at these low powers, not much difference between single or two bay CP installation. However--couple of Class A's in town went to 1/2 wave antennas. Not impressed with coverage from new installation.
If the c-pol antennas had essentially the same r.m.s. gains per polarization, the same radiation patterns, and the same max ERP there would be no practical difference in the coverage they produce, whether they were full or 1/2-wave spaced. Any significant differences would be located on/near the earth within several hundred feet from the antenna site, normally.
Neither. .75 or .8W spaced antennas have less downward radiation than either of the other two. (i.e., full wave or half-wave spaced)
Actually a two-bay array using 1/2-wave vertical bay spacing has less downward radiation than the full wave spaced, and both a 0.75- and 0.8-wave spaced array (see link below).
These patterns are based on a c-pol element with 10% field at +/- 90 degrees elevation, however the gains are shown for linear polarization, and must be divided by two for the h-pol and v-pol gain of a c-pol array.
http://s20.postimg.org/jyxvv923x/LPFM_Pattern_Comparison.jpg
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LPFMs are usually licensed with a horizontal power, but under the rules, they can use any kind of polarization as long as the antenna is omnidirectional. As a practical matter, with the proliferation of automobile window antennas (my '11 Chevy Malibu) or oddball whips (my '05 Ford Focus), I wouldn't use anything but CP on either an LPFM or translator.
OK, here are some practical numbers. I assume an antenna in pool-table country (e.g. corn & beans Midwest somewhere), so antenna height above ground is AHAAT. Thus: 100 ft. tower, 120' of Andrew 1/2 foam for feedline, and a CP antenna, (I used the SWR FM/1 series for gain figures, comparing full wave and half-wave spaced systems):
For 1 bay the gain figure is the same, .441, so a TPO of 273 w is needed for 100 watts ERP
For two bays, full wave, .959; 1/2 wave .695 TPO full wave:126 watts; half wave: 173 watts
For three bays, full wave 1.495; 1/2 wave 1.012 TPO full wave 81 watts; half wave: 119 watts
Theory is fine, but $$ usually rule. A single bay in this example requires a 300 watt transmitter. Go with a two bay, you can use a 150 watt transmitter, while a half-wave spaced antenna will probably mean buying that 300 watt transmitter. At which point you can money by going with the single bay.
Now look at three bays--a bit much for an LPFM, but still makes my point. Full wave spacing would allow use of a 100 watt transmitter; 1/2 wave requires the 150 watt transmitter.
Remember LPFM transmitters must be type certified (believe that is the current phraseology), as opposed to type approved. Limits the available transmitter options.
OK, here are some practical numbers. I assume an antenna in pool-table country (e.g. corn & beans Midwest somewhere), so antenna height above ground is AHAAT. Thus: 100 ft. tower, 120' of Andrew 1/2 foam for feedline, and a CP antenna, (I used the SWR FM/1 series for gain figures, comparing full wave and half-wave spaced systems):
For 1 bay the gain figure is the same, .441, so a TPO of 273 w is needed for 100 watts ERP
For two bays, full wave, .959; 1/2 wave .695 TPO full wave:126 watts; half wave: 173 watts
For three bays, full wave 1.495; 1/2 wave 1.012 TPO full wave 81 watts; half wave: 119 watts
Theory is fine, but $$ usually rule. A single bay in this example requires a 300 watt transmitter. Go with a two bay, you can use a 150 watt transmitter, while a half-wave spaced antenna will probably mean buying that 300 watt transmitter. At which point you can money by going with the single bay.
Now look at three bays--a bit much for an LPFM, but still makes my point. Full wave spacing would allow use of a 100 watt transmitter; 1/2 wave requires the 150 watt transmitter.
Remember LPFM transmitters must be type certified (believe that is the current phraseology), as opposed to type approved. Limits the available transmitter options.
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