I am very curious about the shape of AM antennas and how that shape affects/effects performance.
I am not talking about straight verticals, whether thin wire or thick pipe.
I am very curious about the shape of AM antennas and how that shape affects/effects performance.
I am not talking about straight verticals, whether thin wire or thick pipe.
I am talking about towers with either 4-legs or 3-legs which are wide at the bottom, narrow toward the top, and have all the support braces in triangular patterns all the way up each side. What does that wide-bottom narrow top shape do to the signal?
radio8z says
Antenna Tower Shapes
This is an interesting topic which leads to some technology which is not obvious at first glance. One example is the Blaw-Knox AM tower which is pointed at the bottom and the top and bulges out in the center. It is supported by an insulator at the base and guy wires. The broad center is necessary because of the relationship between the tower dimensions and the radio wavelength where there is low current at the ends but high current in the center of the tower. The large center dimension is to accommodate the high current.
Here’s a link to an interesting site which describes this type of tower:
http://www.j-hawkins.com/blaw-knox.html
Enjoy,
Neil
radio8z says
Who to Believe
After posting above I went to the site I linked and found this:
“There has been some suggestion that the Blaw-Knox tower was designed to be “fat” in the middle to accommodate a current loop (I max) in a 1/2 to 5/8 wave length tower. This was purely consequential and was not a consideration in the design of the tower.”
I do, however, recall reading a rather detailed description of the bulge current and the amount of steel needed to carry this current. Apparently this was on another site which I do not recall.
So it is up to you readers to believe or not this attribute of the tower.
Neil
Ermi Roos says
Fat antennas
A fat antenna will work better than a thin one because the antenna capacitance is higher. This would require a lower-inductance loading coil to resonate the antenna. The lower inductance will reduce the ohmic losses of the coil, improving efficiency.
Fatter on top is better than fatter on the bottom.
Ken Norris says
RE: The Shape of Antennas
“What does that wide-bottom narrow top shape do to the signal?”
If a high power antenna, then not that much I’d say … wider at the bottom probably has more to do with wind stability and a nifty look, like the Eiffel Tower, than signal. I haven’t seen any single towers like that for quite awhile … engineering and build time isn’t worth the expense considering you can use guy cables with standard tower parts.
The diamond antennas are certainly interesting, but also would be more expensive to make and raise.
I’ve thought a lot about building a scale railroad someday, if I could get financial backing. A mini theme park with a scale old time railway system you can actually ride on (a steam engine … boiler fired by stove pellets), a miniature golf course, a puppet theatre, a no-license fishing pond with RC sailboat and electric hydroplane racing, and, of course, a real working (Part 15) radio station with a miniature old style antenna as described.
It’d be a kick in the pants to drive into the park and listen to the activities planned for the day, hear railway schedules, hawk T-shirts, using the character voices and theme music for the puppet shows, etc.
… just a dream … (sigh).
Carl Blare says
Rail Gardens and Shapes
People love small train settings. There is a train installation at our local zoo that goes maybe 1-and-a-half miles through a forest, past animal habitats, into a tunnel and crosses many walking paths with flashing lights and bells.
Having part 15 radio within such a setting would make it really come to life.
Keep following that dream, Ken!
And yes, when I started this thread I took it for granted that wide-based towers were built that way for physical support rather than for some RF advantage, but the question is, what does that shape do to the RF?
Many self-supporting towers continue in operation, perhaps by older radio stations.
A few years ago a local station built “the tallest self supporting tower in the state (Missouri)”, and I am guessing their reason was one of better location but with small sized real estate. Guy wire support requires quite a bit of land. It was an FM tower, not AM.
radio8z says
The Star Tower in Cincinnati
Here’s a link to a rather unique tower. I watched it being built from my office window way back when.
The tower itself doesn’t radiate so the structure was mechanical as well as aesthetic.
http://en.wikipedia.org/wiki/Star_Tower
Neil
Carl Blare says
Appealing Tower
Thanks Neil for the Star Tower link. The design is very different from what is normally seen, and doesn’t look, at least in the picture, as high as it actually is.
The several antennas said to be on the tower aren’t very conspicuous as one would expect. That brings the question of how the antennas are so smoothly incorporated in the design.
radio8z says
Star Tower
The antennae integrate well because this tower is HUGE. The picture doesn’t convey the size. You have to see it to appreciate it.
The station, WSTR, has some interesting history. It went on the air as WBTI ch 64, a subscription station, which used the Oak Industries scrambling system to compete with cable TV by providing the ON TV service and programming. It was quite controversial because, despite the scrambling, a viewer could see a picture and some of the movies they aired were edited porn. Since the programming originated out of state, the Hamilton County Prosecutor could not shut them down. (Simon Leis of Maplethorp and Larry Flint fame.)
There was available a kit with which one could build a descrambler and get free commercial free programming (and porn). The seller of the kit became entangled in a copyright infringement suit with his defense being that since the TV signal crossed his property he was free to do with it what he chose and if BTI wanted to stop him they needed to keep the RF off his property. Sales of the kit did not meet expenses so he stopped selling it and the suit was dropped.
The original transmitter site was co-located with WLWC on Chickasaw Street. (The WLWC tower was the one pictured in the intro to WKRP in Cincinnati.) The TV antenna was moved to a new tower built next to the old red and white tower and WBTI shared this tower. Our ham group had two repeaters housed in the transmitter shed of WLW and the BTI shed was built next door. Having access to the site, I had occasion to chat with the transmitter technician for BTI and learned that their plan was to become a full service UHF station and the subscription part was used to leverage a license. They dropped the subscription service after five years and became WSTR and later moved to the new tower.
I bring this history here because given the meager beginnings it is hard to imagine how the station could afford the Stars Tower. One of my friends owned the dominant two way and paging service in Cincinnati and I suspect that he was involved, perhaps in a financing consortium, since the tower is located close to his facility. I never had the chance to ask him about this.
Neil
Carl Blare says
Side Mounting
Many antennas are side-mounted on a tower, but doesn’t such close proximity to a metal structure have a detrimental effect on radiant patterns?
Isn’t side mounting almost the same as mounting an antenna up against a wall?
radio8z says
Tower Interference
“…doesn’t such close proximity to a metal structure have a detrimental effect on radiant patterns?”
In the VHF and above range the spacing from the tower affects the antenna pattern and sometimes it is used to control the pattern to make the antenna directional. I used this to minimize an intermod problem with a repeater. On two meters the separation between input and output is 600 kHz. There were two repeaters nearby with outputs at 146.700 and 147.300 MHz which mixed in our receiver to give 600 kHz which mixed with our output at 145.270 MHz. to give a product at 144.67 MHz which was our input frequency. This caused severe desense and interference.
A notch filter was installed in the receiver line along with stub traps but these did not totally eliminate the interference so the antenna was side mounted in a fashion to null the pattern towards the other repeaters and the problem was solved. Unfortunately, the repeater was a backup emergency service station and an omni-directional pattern was desired so with more work on filtering and shielding I was able to top mount the antenna, but for a while the tower side mount worked. Many times multiple antenna installations on a tower are not trivial.
We had another repeater at the 600 foot level of the WLW tower I mentioned earlier and it was mounted in the center of the tower which yielded an approximate omni pattern and gave a range of 70 miles radius with 25 watts.
After my move to Columbus, I tended a repeater site with the antenna side mounted on a cell phone tower and it was mounted such that the tower effects were minimized.
On these types of towers it is required that a load analysis be done to ensure safety and the tower owners were kind enough to donate the cost of this.
But, I digress….
Neil