FM antenna – Page 3 – Engineering Radio

The Shively 6810 FM antenna

Update, W232AL:

The news is out; this is for the new “WFAS-FM” which is actually W232AL retransmitting the WPLJ HD-2 channel. What do they call translators these days… Metro stations? Something like that. Anyway, quite a bit of work went into getting this off the ground before the start of Labor Day weekend and here it is!

We are currently working on a project that involves installing a Shively 6810 FM antenna. Since few people get to see these things up close, I thought I would post a few pictures.

This particular antenna is a four-bay, half-wave spaced directional antenna. It is going to be side mounted on a 430-foot tower. To do this, we had to lower the AM skirt wires by about fifteen feet and retune the AM antenna.

This Shively antenna came in seventeen boxes with sixty-four pages of assembly instructions. There are many parts and they need to be assembled in the order specified, otherwise, things get in the way. We found that Shively provided many extra bolts, washers, O rings, etc because things get lost. Also, all of the parasitic locations and bay orientations were clearly marked. One thing that the tower crew said; always check the Allen screws and other hardware on the elements before installing the RADOMES.

Since this is a half-wave antenna, the radiating elements are 180 degrees out of phase, bay to bay.

Stainless steel tower leg mounting brackets.

Assembled element with RADOME. This is the top bay with the gas pressure release valve

We hoisted two bays at a time. The top bays are ready to go up.

The bottom two bays were hoisted next.

This is the antenna installed, less the tuning section and parasitic elements. It is tilted off axis from the tower by design due to its highly directional nature.

The transmission line was installed and we swept the antenna. I will snap a few final pictures once the transmitter is installed, which will happen tomorrow.

Updated Pictures: Here are a few pictures of the finalized installation:

W232AL antenna, new installation on WFAS AM tower

The fully installed antenna, tuning unit and transmission line. We did some program testing, made sure the audio sounded good, then the station was signed on. We also had to lower the AM station’s skirt a few feet and retune the ATU. Actually, the ATU needed to be reconfigured because the shut leg had been disconnected and there was a capacitor added to the circuit after the base current meter. All of that was fixed, along with a few other things…

W232AL transmitter, a BW Broadcast TX300 V2

The W232AL transmitter is a BW Broadcast TX300 V2. These little transmitter are packed with features like a web interface, on board audio processing, etc. They are pretty neat.

Its Friday, time to go home! — It’s Friday, time to go home!

The tower crew from Northeast Towers did a great job, as they always do.

The Shively 6710 Antenna

Perhaps that is one Shively Antenna that you haven’t heard of. They were an oddball combination of a horizontally polarized antenna with an adjustable vertical element. This design allowed the station to adjust the ratio of horizontal to vertical power from a range of 1:1 to about 4:1 (H:V). Why would this be a desirable feature?

Back in the early days of FM broadcasting, almost all stations had horizontally polarized antennas. This system worked remarkably well, stations could broadcast at moderate power levels over fairly long, line-of-sight (or mostly line-of-sight) paths. Most FM receivers were stationary units installed in people’s homes often with outdoor antennas.

It was not until the late 1960s and early 1970s that FM radio receivers became a stock option in most low and mid-cost automobiles. It was then that a slight problem with FM broadcasting was discovered; car antennas are vertically polarized. People driving around in their new machines found that the FM reception was not all that great. Stations began adding a vertical component to their signal to help improve the mobile reception situation.

I found this Shively Brochure in a file cabinet drawer at the WFLY transmitter site. This model antenna was ordered and installed by that station in 1970. It had a 3:1 horizontal-to-vertical ratio. Why not install a fully circularly polarized antenna? Because often that necessitated installing a new, more powerful transmitter. Every watt of power taken from the horizontal plane and added to the vertical plane reduced the ERP by that much and had to be made up with more transmitter power output. Oftentimes, the ratio of H:V power would be adjusted to take up whatever headroom there was in the transmitter and the station would run that way until the next transmitter replacement cycle.

I found the remains of this antenna in the woods, northeast of the tower.

This section looks pretty well destroyed. It is probably better to dispose of these types of things by scraping, them rather than dumping them in the woods. While there is not a lot of scrap value to this unit, it can become attractive nuisance to copper thieves and other vandals if it is left laying about.

It is a strange-looking piece of kit, a sort of make-do until the situation could be fully rectified. I think this antenna was in service until 1986 or 87 when it was replaced with a circularly polarized ERI.

Filing an STA

FCC rules stipulate that when a station is operating at variance from its licensed parameters for more than 10 days, Special Temporary Authority (STA) is required. The reasons for requesting an STA are varied but could include things like:

Damaged transmission equipment
Loss of transmitter site or building use
Loss of tower
Eviction
Facilities upgrade or renovation
Natural disaster

The loss of the transmission tower at WUPE-FM falls into one of those broad categories. Thus, we have filed an STA with the FCC for temporary transmission facilities while a new tower is being constructed. Since the old tower is completely lost, we specified a new tower location, new height above average terrain (HAAT), new ERP, and environmental certification. To gather that information, several steps were needed:

Obtain a new tower location. This was done with a GPS receiver and verified on itouchmap.com. Once the NAD83 position was obtained, it needed to be converted to NAD27 for the FCC filing. The FCC has a conversion tool on its website.
HAAT calculation is fairly simple, use the HAAT calculator tool on the FCC website. For this, the antenna radiation center height Above Mean Sea Level (AMSL) is needed. Using a topographical map, find the ground level AMSL, convert it to meters, then add the radiation center height above ground level (AGL).
The Effective Radiated Power (ERP) calculation is also simple; Transmitter Power Output (TPO) minus system losses (transmission line and antenna gain). It is easiest to do this in dBm, e.g. convert the TPO from Watts to dBm, then add or subtract the gain or losses in dB, and convert the final product back to Watts.
The environmental statement is slightly more tricky. Basically, the filer is certifying that the STA complies with all environmental regulations including OET-65 (RF exposure limits). Since the temporary antenna is significantly lower than the original, some investigation is required. For this, there are two methods to demonstrate compliance; ground measurements with a NARDA meter, or RFR worksheets which are a part of the broadcast station renewal form, FCC-303s.

I have taken the RF worksheet sections out of the 303s and separated them into the FM RF Worksheet and the AM RF Worksheet. These worksheets are not effective for large tower farm-type sites where there are too many variables and RF contributors to be accounted for. The calculations on the worksheets are not conclusive, however, if the facility in question falls under the limits, it is generally accepted as being in compliance. Taking ground measurements with a NARDA meter is the definitive method for determining RFR compliance. Since this is a relatively simple site, the worksheet calculations should be sufficient.

The worksheet calculations show that the RFR is within both the controlled occupation limits and the uncontrolled general population limits.

The position of the new temporary pole was verified on itouchmap.com:

It is never good to be operating at a varience from licensed parameters without notification of the FCC. Such things could lead to fine or other problems for the broadcaster.

The unitless coefficient of Zorch

Zorch is a term used to describe an over voltage or over current condition that usually leads to catastrophic failure, e.g. the power supply was zorched by lightning. There is also a quality to radio signals that defy and exceed theoretical definitions for service contours or power density. That is quality defined as:

Zorch (adj): The ability of an RF signal to be received in unlikely locations; outside of predicted service contour, in steel structures, underground facilities, tunnels, etc.

It brings to mind the saying, “antennas are not amplifiers and amplifiers are not antennas.”

During the earlier stages of FM broadcasting, there was a notion that costs could be reduced by increasing antenna gain and reducing transmitter size. While theoretically, ERP (Effective Radiated Power) is ERP, broadcasters soon learned that high gain antenna, low TPO (Transmitter Power Output) installations lacked building penetration and had other reception issues. Realizing that there is a trade off between antenna bays, transmitter power output especially in difficult reception areas, a great debate occurred and continues on what the optimal system is. The answer is, it depends on the receiving environment.

Where this technical detail can be really important is with lower powered FM stations; Class A and LPFMs to be exact. They are already battling against bigger stations that have tens or even hundreds of times more power. Certainly an LP-100 station has it’s work cut out for it. The choice of antenna is perhaps one of the most important technical decisions to be made. Choosing the right balance of antenna type, antenna gain, antenna height and transmitter power output can greatly influence reception reliability and thus coverage area.

A good study of this quality can be had by looking at various LPFM installations:

Station ERP (watts)	Antenna Type	Antenna Gain (power)	TPO (watts)*	Coefficient of Zorch
100	1 bay vertical	0.92	127	0.1
100	1 bay circular	0.46	253	0.4
100	2 bay vertical full	1.98	58	0.15
100	2 bay vertical half	1.40	83	0.2
100	2 bay circular full	0.99	118	0.5
100	2 bay circular half	0.70	166	0.7
100	3 bay circular full	1.52	77	0.46
100	3 bay circular half	1.01	115	0.52

*Includes 100 feet of 1/2 inch foam transmission line, Andrew LDF4-50A, loss of 0.661 dB at 100 MHz, or 0.859 power gain.

Stations should try to get the transmitting antenna as high up as permitted without reducing ERP. In other words, the FCC allows 100 watts ERP with 98 feet Height Above Average Terrain (HAAT) radiation center in their current LPFM rules. Being lower in height will reduce the coverage area. Going over 98 feet HAAT will cause the station’s power to be reduced, which will lower the coefficient of zorch accordingly. Therefore, getting as close to 98 feet HAAT, which is different than 98 feet above ground level in many places, will net the best performance.

If a singular polarization (horizontal or vertical) is desired, vertical polarization should be chosen, as most mobile reception is by a vertical whip antenna. For best reception performance, a circularly polarized antenna will work best, as receiver antenna orientation will not effect the signal reception. A circularly polarized antenna has better building penetration and multi-path characteristics. The FM broadcast circularly polarized antenna in not a true circularly polarized antenna, it is actually unpolarized.

The use of a multi-bay antenna has the effect of focusing the RF radiation outward, perpendicular to the element stack, thus limiting the radiation directly up or down from the antenna. This is more pronounced with one half wave spaced antennas, which may be an environmental consideration in heavily populated areas.

Thus, the best coefficient of zorch for an LPFM station would be a circularly polarized, 1/2 wave spaced, 2 bay antenna. This antenna would have some gain over a single bay antenna, take up less room on a tower than a full wave spaced antenna, offer good RF protection performance for the general public living and working under the antenna, reduce wasted upward radiation and offer good building penetration for the ERP. It would require a slightly larger transmitter and more electricity, but that trade off is well worth the effort.