Folded unipole antennas - why so few?

[rbrink]

I've always been curious about why folded unipole antennas for AM broadcasting have never been more popular. They seem to have various advantages over common series-fed monopoles. No heavy ceramic base insulators required, no isolation transformers for tower lighting required, wider bandwidth than common monopoles, and overall same performance as monopoles. Also they seem more suitable for supporting auxiliary service antennas since they would not need isolators at the base either.

What am I missing?

 

[davideduardo]

I've always been curious about why folded unipole antennas for AM broadcasting have never been more popular. They seem to have various advantages over common series-fed monopoles. No heavy ceramic base insulators required, no isolation transformers for tower lighting required, wider bandwidth than common monopoles, and overall same performance as monopoles. Also they seem more suitable for supporting auxiliary service antennas since they would not need isolators at the base either.

What am I missing?

Harder to tune for directional systems. Very practical for non-directional stations, though.

I've never seen a DA with unipoles. Do any exist?

 

[Kelly A]

I've always been curious about why folded unipole antennas for AM broadcasting have never been more popular. They seem to have various advantages over common series-fed monopoles. No heavy ceramic base insulators required, no isolation transformers for tower lighting required, wider bandwidth than common monopoles, and overall same performance as monopoles. Also they seem more suitable for supporting auxiliary service antennas since they would not need isolators at the base either.

What am I missing?

Unipoles have been controversial. Some consultants feel they're viable alternatives for towers who aren't insulated, potentially the wrong resonant length, have other leased services like FM, land mobile on the same tower, or have compromised ground systems. And generally that's a correct assumption. Reality is; as compared with properly designed, dedicated tower, broad-banded ATU, and a proper ground system, unipoles offer no advantages, and in many cases are less efficient with a much higher skywave than a conventional antenna/tower.

 

[Kelly A]

Harder to tune for directional systems. Very practical for non-directional stations, though.

I've never seen a DA with unipoles. Do any exist?

I've seen a couple. Usually lower power, super low budget, two radiator, where a unipole was added to gain a night DA pattern.

 

[davideduardo]

Unipoles have been controversial. Some consultants feel they're viable alternatives for towers who aren't insulated, potentially the wrong resonant length, have other leased services like FM, land mobile on the same tower, or have compromised ground systems. And generally that's a correct assumption. Reality is; as compared with properly designed, dedicated tower, broad-banded ATU, and a proper ground system, unipoles offer no advantages, and in many cases are less efficient with a much higher skywave than a conventional antenna/tower.

I had a unipole at WDSR in Lake City, FL, a Class IV. They extended the tower to get good FM height, but made the AM unipole because it was "easier" than coupling the FM on an insulated tower. In addition, it was in a lake and had a deteriorated ground.

Given the conductivity of natural rain-replenished lakes, I don't know whey they spent so much on the tower in the water; they had ample land for it. Perhaps someone did not know that there is a difference between fresh and salt water...

 

[bigdonho]

Harder to tune for directional systems. Very practical for non-directional stations, though.

I've never seen a DA with unipoles. Do any exist?

WWNN in Pompano Bch FL had two towers tht were folded unipoles in a 6 tower array. The only real problem was no MOM proofs on arrays with folded unipoles.

 

[rbrink]

WWNN in Pompano Bch FL had two towers tht were folded unipoles in a 6 tower array. The only real problem was no MOM proofs on arrays with folded unipoles.

What are MOM proofs?

 

[boiseengineer]

I know of a couple DAs using them.
One I took care of had 1 1/2 wl unipole and 3 more conventional 1/4 wl towers.
Stable unless really wet or even just frosted up.
Only thing on it besides the unipole is an STL antenna.

 

[Kelly A]

What are MOM proofs?

Method of Moments. It's essentially a series of calculations instead of going out in the field and doing potentially dozens, for a partial proof, or hundreds for a full proof- of field measurements.

 

[Schroedingers Cat]

I suspect that DAs have an even bigger problem with top loaded and sectionalized towers. I remember for years KELO 1320 stated that it was "Franklinized to 13,600 watts during Daytime operation" in the SRDS directory, for which Daytime operation was nondirectional. Also, maybe the Chicago Area participants remember why both WIND 560 and WEAW...WKTA 1330 went from top loaded to somewhat taller monopoles when they moved their arrays to nearby properties.

Found a link about KELO, on Page 4.

https://worldradiohistory.com/Archive-US-Radio/61/U-S-Radio-1961-Mar.pdf

Two tower arrays with lower field ratios in the second tower, say <0.5, and not that critical, having very shallow nulls, are probably good candidates for folded unipoles. However, in some cases, if they did radial measurements instead of Method of Moments, they might find that the conductivity is such that they may not need a DA, or if they do, much shallower nulls in the pattern. WION in Ionia, MI measured many radials, and went from 5000 watts with a three tower array with fairly deep nulls, to 4700 watts nondirectional. They still use the three tower array for 330 watt Night operation, one of a few that got automatic Class B operation by FCC calculations in the 1980s, but for which rules have changed, so they left it alone. WUFL...WKEG 1030 Sterling Heights, MI immediately was able to let out nulls after the original Proofs in 1988, and then much later went to a three tower array with the major lobe toward WTKA 1050 Ann Arbor, MI, again by using the original proofs.

 

[SomeRadioGuy]

I've always been curious about why folded unipole antennas for AM broadcasting have never been more popular. They seem to have various advantages over common series-fed monopoles. No heavy ceramic base insulators required, no isolation transformers for tower lighting required, wider bandwidth than common monopoles, and overall same performance as monopoles. Also they seem more suitable for supporting auxiliary service antennas since they would not need isolators at the base either.

What am I missing?

They dont like ice. Worked at a station in PA that uses one and it would ice up and cause the transmitter to cut back to 1/3rd power

 

[secondchoice]

I wonder does several inches of snow help or hurt ground conductivity? I have never worked at an AM in "snow county". KIUL had amazing coverage with a shunt fed system thanks to ground conductivity but it hardly snowed when I was there.

 

[frankberry]

I would imagine that a few inches of snow might have a small effect on ground conductivity.

 

[davideduardo]

I would imagine that a few inches of snow might have a small effect on ground conductivity.

Of course, rain or snow are, except for contaminants they pick up on the way down, close to being like distilled water. I'm guessing that snow might have a tiny positive effect if the area itself has very low conductivity.

We have different conductivity even in each of the fresh water Great Lakes, so this would be an interesting subject to study were anyone concerned about AM any more. I know some friends have told me preferred to do "points" and, particularly, measurements for directional proofs, when it had not rained for a few days at least.

 

[Schroedingers Cat]

There are ionic salts in the soil, which when they become wet, become a solution that conducts the signals better. There are also ionic effluents running into the Lakes. Notice that Lake Superior is the least, and Lake Erie and Lake Ontario, are the greatest. With the water cycle, as you get closer to the open ocean, more and more salts are deposited. The Baltic Sea has a conductivity of 1000 mS/m, whereas the open ocean is 5000 mS/m, more or less.

Pure water has a very low conductivity.

 

[secondchoice]

I thought snow was formed around a partical of dust. And once it hits the ground I am pretty sure it's not pure. I have never taken any measurements but I doubt anything could make ground conductivity worst between Chattanooga and Atlanta. We really don't get enough snow regularly plan on getting verifiable measurements.

 

[kevtronics]

1170 WWTR in NJ uses one. Non-directional 600-watt daytimer.

 

[davideduardo]

I thought snow was formed around a partical of dust.

"Snow forms when tiny ice crystals in clouds stick together to become snowflakes. If enough crystals stick together, they'll become heavy enough to fall to the ground"

Google Search

Or...

snow

Tiny crystals of ice that fall to Earth are called snow. A crystal is a solid substance that has flat surfaces and sharp corners. Snowfall is made up of both single ice…

kids.britannica.com

 

[MM797]

Harder to tune for directional systems. Very practical for non-directional stations, though.

I've never seen a DA with unipoles. Do any exist?

I saw a three tower array in Florida, north of Orlando, in around 2002 that used those tapering corrugated galvanized pipe style free stander towers that are popular for cellular phone towers. It was right next to a railroad track so maybe the rails were bonded to the ground system and the rails ran toward Orlando so perhaps the ground conductivity worked good in that direction.

 

[SomeRadioGuy]

I saw a three tower array in Florida, north or Orlando, around 2002 that used those tapering corrugated pipe style free standers that are popular for cellular phone towers. It was right next to a railroad track so maybe the rails were bonded to the ground system.

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