I've seen this happen so many times, it's time to give it it's own thread. Budget brands of directional FM antennas (PSI, SWR, Nicom, etc) will often have non existent coverage in many directions. One case in point had a station operating non-d at less than 3KW providing a listenable signal at my location (30 miles +/-). When they went to about 10x the power DA, with a signal that was predicted to show an improvement my way, the signal literally disappeared. The budget antennas probably do fine in the main lobe direction and provide nulls where required, but as far as filling out the authorized coverage pattern, I've seen an endless parade of disappointments. Think long and hard before passing on an ERI or Jampro bay time antenna with parasitics. You get what you pay for.
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Directional FM Antennas
- Thread starter BobOnTheJob
- Start date
The late great chief engineer Bill Sanderson sent the pattern for WWBN Tuscola, MI back to the designers three times before they got it close to being right. But I don't think that was the low budget model. You have to really bird dog the designers to get it anywhere near close to right. I have a friend that has an full scale antenna testing range on several acres. I don't know if he has time to design antennas for others, but I'll ask him if you want me to.
If you think about it, the licensed envelope for many patterns would require many elements to achieve. Think of what AM DA Patterns look like. Though not achieved by driven phased elements, FM parasitic elements show many of the same vectorial characteristics. But the actual pattern only has to be 85% RMS of the licensed pattern as I recall, and that would be only 70% of the pattern area. A station owner once told me that FM DA manufacturing and testing is a lot like sausage manufacture-you don't want to know what really happens on the testing range.
I know of a few FM DAs for 6 kW As and 25 kW B1s and C3s that are a complete disappointment. With 3 kW Class As you're often better off just mounting the Omni antenna in an advantageous way than going to 6 kW directional, at least signal wise in the desired directions.
If you think about it, the licensed envelope for many patterns would require many elements to achieve. Think of what AM DA Patterns look like. Though not achieved by driven phased elements, FM parasitic elements show many of the same vectorial characteristics. But the actual pattern only has to be 85% RMS of the licensed pattern as I recall, and that would be only 70% of the pattern area. A station owner once told me that FM DA manufacturing and testing is a lot like sausage manufacture-you don't want to know what really happens on the testing range.
I know of a few FM DAs for 6 kW As and 25 kW B1s and C3s that are a complete disappointment. With 3 kW Class As you're often better off just mounting the Omni antenna in an advantageous way than going to 6 kW directional, at least signal wise in the desired directions.
The only way I've had good results with a DA is going with panels. But that can be expensive and if your tower won't support them, impossible.
Take the trip out to the range and see what's going on for yourself. It's worth it. And if you're getting a DA without a proof on the test range with your tower, feedline and mounting factored in, you're just making an educated guess.
I have a weird DA comprised of 4 Nicom antennas, and a mounting pole at one translator site. Was extensively modeled using NEC prior to installation. Seems to work, but has never been on a test range.
Take the trip out to the range and see what's going on for yourself. It's worth it. And if you're getting a DA without a proof on the test range with your tower, feedline and mounting factored in, you're just making an educated guess.
I have a weird DA comprised of 4 Nicom antennas, and a mounting pole at one translator site. Was extensively modeled using NEC prior to installation. Seems to work, but has never been on a test range.
I've had discussions with antenna designers and professional engineers several times on this issue. Though there could be QC problems with some antennas, it's not an issue confined to budget brands. The consensus is that omnis often radiate far, far better than predicted while DAs are very carefully controlled to avoid that. The net result is that DAs frequently do not perform as well as omnis, even when predicted to do so.
The best advise seems to be not to use a DA unless there's just no other way.
The best advise seems to be not to use a DA unless there's just no other way.
Contributing to the differences noticed for DA coverage even at the peak of the pattern is that a DA is licensed on the maximum power permitted at that peak (while meeting its protection requirements), while non-D ERP is licensed on its r.m.s. value.
Pattern distortions caused by the mounting environment can mean that even though a DA and a non-D both are licensed for 6kW, the ERP of the non-D could exceed that at the peak gain of the DA, by several decibels.
RF
Pattern distortions caused by the mounting environment can mean that even though a DA and a non-D both are licensed for 6kW, the ERP of the non-D could exceed that at the peak gain of the DA, by several decibels.
RF
If you are a 3 kW Class A, in some cases, interference issues as 6 kW Class As can sometimes be solved by reduced ERP from 6 kW at 100 meters HAAT, sometimes by the equivalent full power above 100 meters, and sometimes with 6 kW at below 100 meters. In any case, 4-5 kW omni at 100 meters is a no brainer signal wise vs. 6 kW directional. And if you're the equivalent of 4-6 kW at 100 meters at above 100 meters, even the owners and managers and advertisers may not realize that it is not the full equivalent ERP.
Schroedingers Cat said:
ERI used driven phased elements in the DA-1000 series. They used two driven horizontal elements (w/ two parasitics) and one driven vertical element (w/ 3 parasitics) per bay. I took care of a DA-1005 (5 bay) for several years. The 3-way power divider was a weak point - a failure in any of the driven elements would often eat the power divider. I had that happen twice in my years there, and it happened at least a few more time before the antenna was replaced. The directionalized rototiller would seem to be a simpler and more reliable design.
Dale H. Cook said:
I should have put "usually" in that sentence.
I have often wondered about how practical a stacked array of bays spaced and phased like an AM DA or CATV and MATV head end antennas would be. They would have to be in a staggered arrangement, and also mounted so that tower or support interactions would be minimal to have maximum flexibility. I know some FM DAs use phased Yagis pointed in different directions, but these are usually used for translators and lower power NCE-FM stations with complex directional requirements. Yagis would not be good for situations where you really want a pattern as close to Omni as is practical.
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Re: Directional FM Antennas note :
ant monted on the face of a tower is going to have 3 db gain vert and wider the face gets the closer the Horz comes in line with the foward gain thats with a full wave spaced a lot of ant desiged is at scale modeling four time the feq the better test is out side in full field testing but some of these station almost run there full power up to the ant with 60 thos watt trans I dont every thing but that being said is offton the Broad telling the FCC what there going to be doing 40 thous up real high is going out there .........................best tools is in a small low power small number of bays getting a good long dist ..................................
ant monted on the face of a tower is going to have 3 db gain vert and wider the face gets the closer the Horz comes in line with the foward gain thats with a full wave spaced a lot of ant desiged is at scale modeling four time the feq the better test is out side in full field testing but some of these station almost run there full power up to the ant with 60 thos watt trans I dont every thing but that being said is offton the Broad telling the FCC what there going to be doing 40 thous up real high is going out there .........................best tools is in a small low power small number of bays getting a good long dist ..................................
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