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Does your choice of antenna polarization have an affect on your stations range?

 

This question refers to the physical orientation of the antenna elements and is categorized as "Vertical Polarization" (VP), "Horizontal Polarization" (HP) or a combination of both which forms "Circular Polarization" (CP).  Some may be surprised to learn that the actual type of polarization used in the FM broadcast band has no direct impact on how far your signal will reach.

 

It is the stations "Effective Radiated Power" (ERP) combined with the antennas "Height Above Average Terrain" (HAAT) that determines range. You can confirm this by looking up the coverage map of any station licensed to run equal ERP levels in both vertical and horizontal polarization. The coverage area for the horizontal signal overlaps the same area the vertical signal has and does not add to the range in any way.

 

The best range and most efficient use of your "Transmitter Power Output" (TPO) occurs when the  polarization of the receiving antenna matches that of the transmitted signal it is picking up. We now must consider the antennas that our listeners are using on today's receivers. It's important to note that just about every receiver manufactured in the last 2 or 3 decades now uses a 75 ohm antenna system.

 

This has nearly eliminated the sales of any indoor receivers that were equipped with the old fashioned 300 ohm folded dipole antenna which looked like a "T" hung on the wall horizontally. The main problem with horizontal receiving antennas shaped like this is they are only sensitive to signals coming from the two directions that are broadside to the "T" and nearly deaf off both ends or tips of the "T".

 

The modern 75 ohm system uses the line cord on inexpensive receivers or a resonant, 1/4 wavelength piece of wire as the antenna on better ones. Both of which are often hanging vertically out of the back of the indoor receiver. The main advantage to vertical receiving antennas is they are sensitive to signals arriving from all directions. This is also the reason almost all portable and vehicle radios use vertically polarized antennas. Otherwise there would be extreme changes in signal level every time the receiving antenna was turned.

 

Those of us old enough to remember when cars had this "T" shaped horizontal antenna imbedded in the windshield can vouch for this problem being present each time you turned a corner while listening to a distant FM station. Since  AM broadcast stations only transmit with vertical polarization and the AM receiver shares the same FM antenna in your vehicle, these horizontal antennas were unresponsive to all but the strongest AM stations.

 

Most listeners today are in their vehicles and those antennas have evolved in a more interesting way.  Taking on new shapes that look like small domes or shark fins on the roof. In some cases the familiar whip is still used but much shorter because it has been loaded. Sometimes we see it swept backwards rather than perfectly vertical. Are they using horizontal, vertical or both polarizations? They are still most sensitive to the signal arriving in the vertical polarization. 

 

Even if the receiving antenna were completely horizontal, as soon as you place it directly on top of a flat metal surface like the cars roof, this forces the polarization to reflect upwards into the vertical plane. This also removes most of the directional characteristics associated with  horizontal receiving antennas. Their small size and low gain is often compensated by the use of RF preamps and receivers with improved selectivity. CP antennas are not used for reception in FM because that would remove the advantage this type of polarization could offer for reasons discussed later.

 

You may now ask since there are no CP antennas manufactured for use in FM reception, what advantage could transmitting in CP offer your station when no receiving antennas will match this polarization? The most common answers are "better penetration in steel framed buildings" or a "reduction in multi-path interference".  Let's look at these one at a time and then we can consider what this simple explanation may have overlooked.

 

First, VHF frequencies used in the FM band have virtually no ability to pass through metal. Try taking a portable FM radio inside an elevator and it works just as good as your cell phone no matter which polarization or direction you turn the antenna in. If something has blocked the signal, you must move the receiving antenna to a location where the signal is not blocked before you can worry about which polarization it may be arriving in.

 

In most cases where a signal is blocked by metal we can move the receiving antenna as little as a few inches and the station returns. You may have observed this in your car at a red light and pulled forward just a bit to continue hearing that song. The same thing can be noticed inside a steel framed building by moving that antenna or radio a few inches. If reception has not been totally blocked  inside a metal structure or through one, the signal has either passed by it or in some instances has reflected off of it. 

 

It's when the signal is reflected rather than blocked that you may notice the true advantage with CP.  When a signal reflects off metal and bounces back to your receiver antenna it can distort the polarization and phase of the signal much like a mirror reflects things backwards. This can create "multi-path distortion". Since CP disperses your transmitters power into all polarizations rather than concentrating it into one, you may not have to move the radios antenna to pick up a signal that was just distorted by reflections and not totally blocked. 

 

Since a circular polarized signal is actually spinning in a helical spiral as it propagates, it also has a direction of rotation. Either left handed or right handed. When the signal reflects off some object, this direction of rotation is also inverted. Not perfectly matching the polarization is far better than the signal loss of perfectly mismatching the polarization like this would cause. This is why CP antennas are not used on vehicles since multi-path reflections would change the direction of rotation. 

 

At the end of reading this some stations will conclude that CP is still their best choice so we should discuss "True CP" versus "Mixed Polarization" (MP). Antennas with a horizontal loop in the center and tips that extend out like a vertical dipole are not true CP antennas. They are a mixture of horizontal and vertical polarization rather than all polarizations. They often suffer from a high "Q" with a narrow bandwidth that is easily de-tuned in bad weather conditions. If CP is your choice, I recommend you pick a model that uses a symmetrical pair of crossed and tilted elements to avoid these drawbacks.

 

We have covered a lot so far and are just about to get to the most important part on how all of this does affect your stations ERP and range. From what you've read this far, you understand CP has a slight advantage in that it can reduce multi-path interference and provides improved reception to older horizontal style antennas that may still be in use. Then just buy those expensive CP antennas, right?

 

When it comes to antennas there is almost always a trade off between gaining one benefit at the expense of another. It simply is not technically possible to construct one antenna that can provide all of the benefits found in various designs. In order to form a real CP signal you must split your transmitters power output between two matching elements in opposite polarizations and one must lag the other by 90 degrees in phase.

 

This causes a major reduction in the Power Gain of the antenna system and no FM broadcast CP (or MP) antenna can provide a power gain of more than 0.5 or -3dbd. Since the Effective Radiated Power of your station is equal to the amount of watts your transmitter can deliver to the antenna, multiplied by that antennas Power Gain, it becomes easier to understand you've lost half of your ERP and that will absolutely reduce your coverage area if it has not been compensated for.

 

For example, if your transmitter can supply 1000 watts to one CP antenna with a Power Gain of 0.5, your stations Effective Radiated Power will only be 500 watts. On the other hand, that same 1000 watts feeding one NWE-34 with a Power Gain of 2 would be producing an ERP of 2000 watts. Stations that have simply replaced one CP antenna with the NWE-34 typically report that this 4 fold increase in ERP has produced approximately twice the range as a result. Twice the range encompasses 4 times the coverage area!

 

To produce the same 2000 watt ERP with a CP antenna would require a transmitter to operate at no less than 4000 watts. Failing to compensate for the 0.5 Power Gain (-3dbd) associated with all CP antennas would not only eliminate any slight advantage this type of polarization could offer, it would severely compromise your overall range by providing only 25% of the total ERP as compared to the NWE-34 antenna. You can't begin to compare CP against VP until you're able to equal the same Power Gain or ERP in both examples. 

 

I've never seen any unloaded antenna able to produce a lower Power Gain than the 0.5 found with omni-directional FM broadcast CP antennas. They have the least efficiency of all options and the proof is almost none of the big Commercial stations would even consider using an antenna system with a Power Gain of 0.5. Could you imagine trying to feed a 100,000 watt transmitter just to reach 50kw ERP? Choosing a system that provides positive gain can cut these requirements to a small fraction of this TPO.

 

Even with the big profits commercial advertisements generate, you usually won't see them run anything less efficient than an antenna system with a Power Gain of 2. The only way to achieve a Power Gain of 2 using CP antennas is to stack four of them over a 40 foot section of tower and feed them through a power divider with four inter-bay cables. The expense of the power divider alone can easily exceed the cost of the antennas.

 

If the stations finances permit all of these extra expenses, years of experience have also provided an education on the long term reliability of well constructed antenna systems. The weakest link in the chain is usually going to be all of the RF connectors in use on the tower or mast. It could be water migration, lightning damage, RF heating or Corona arcing, the connectors are often the most vulnerable point. 

 

One NWE-34 antenna places just two RF connectors outside of easy reach. One on the antenna and one on the end of the transmission line. Consider that four CP bays typically use 18 RF connectors on the tower! Ten on the four port power divider and another eight on the four CP antennas. Then we have the issue of snow and ice loading on these antennas that are all hanging about 3 feet off the same side of the tower.

 

It's not just a problem from the added stress this off-centered weight places on the support structure. I've seen several cases where ice from an upper CP bay has fallen on a lower bay and broken these fragile elements. Unfortunately this causes an imbalance in the system through the power divider and results in excessive reflected power that shuts the transmitter down even though the other antennas were not damaged. The same situation exists when any connector fails.

 

Looking deeper into the polarization topic will reveal that in the USA it's mostly antiquated regulatory issues by the FCC which mandated that commercial FM stations maintain compatibility with older horizontal receiving antennas that has promoted their use of CP. The FCC only imposes this restriction on the primary antenna of a commercial FM station while excluding all other applications from auxiliary to LPFM. Nearly every other country allows or requires all of their FM stations to take advantage of the efficiency that can easily be obtained using just vertical polarization.

 

It is the ERP and location the antenna is mounted in that determines an FM stations range, not the particular type of polarization used. Increasing your ERP four times over one CP antenna picks up many more listeners with its expanded coverage area when compared to just filling in the horizontal coverage within an existing range or ERP level. Vehicles traveling across the coverage area of the NWE-34 can notice they hear the station twice as long when compared to any CP model. This range is based on having similar terrain throughout the coverage area and with the same power level being applied to both antennas.

 

The benefits of CP are usually only noticed by a small percentage of listeners but come with both large upfront expenses for equipment and long term expenses for electrical consumption or leasing a huge section of prime tower space to stack many CP antennas. This is why big 50kw commercial stations use CP. They can cover the expense that this four fold reduction in efficiency causes since that is easily earned with just a few commercials and their commercial FCC license forces them to include HP.

 

For these same reasons smaller stations and non-commercial stations are likely to find the high gain and simple installation of the NWE-34 to be the most effective option to achieve their maximum ERP and range economically. The type of polarization used simply determines the orientation a receiving antenna can be in while still providing good reception for distant or weak signals. Local or strong signals are still easily received even when the reception antenna is of a different polarization than the transmitted signal.

 

Once CP is at the same ERP as VP, you should have some improved reception to vehicles that may be experiencing multi-path distortion. In urban areas with an increased possibility of reflections, you may notice CP has less momentary signal fluctuations as you drive through these reflections. They are typically very brief, lasting a fraction of a second and do not cause the listener to change channels. In these cases CP may provide a signal that is a bit more consistent but not with increased range or distance. 

 

In most urban cities the people who work in the tall steel framed buildings tend to live and commute to the suburbs. The transmitter site is often in the same city where it provides a very strong signal able to penetrate most buildings. It's also much more difficult to cause the complete loss of a strong local signal. Now what if there were a legal way to provide more range to those listeners in their cars during the commute?

 

If the NWE-34 does not have to be side mounted, coverage to vehicles will be expanded in most directions even when the same ERP levels are used. It's easy to underestimate the side effects that all side mounted antennas suffer from and it can be much worse for CP antennas. The mast or tower will obstruct, reflect and distort the pattern of an antenna that is mounted parallel to it. The angled lattice work on towers causes reflections that are very difficult to predict or compensate for when the signal is in all polarizations. 

 

Being able to remove this obstruction from the radiation pattern makes a noticeable improvement to vehicles traveling in the stations fringe coverage area. In these weak signal locations the advantage is a consistent reduction in white noise or static as opposed to the momentary improvement CP could offer when just passing by a location of multi-path distortion. You're not required to reduce output to offset this advantage and it's only when the static becomes more consistent that the listener changes the station. 

 

No other antenna is guaranteed to produce the most coverage per watt (or your money back) because no other antenna is able to provide more coverage area per watt. If your choice is between CP or 400% more ERP with the NWE-34, always choose the NWE-34 antenna. Regardless of polarization, you can expect a signal that is 4 times stronger to provide better penetration and coverage than one that is 4 times weaker. 

 

To be transparent, when the power, tower space, funds and license requirements are in place to operate CP at the same ERP level of a compared VP system and top mounting the antenna is not possible, I would then consider the CP option. Once you're able to equal the ERP that the NWE-34 produces, you can then afford the inefficiency associated with filling in horizontal coverage over the same area. You're well informed now. If you have any questions please feel free to use our contact page information to call by phone or send email.

 

 

   

   

 

 

 

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