It has been over 30 years since I first started as a DJ at the age of 13 on WLPJ radio in New Port Richey, FL. I was very in-experienced at that time but determined. After many years in this field and many problems encountered, I believe I have acquired the necessary skills to give some advice to LPFM stations that are growing in the Country.
Setting up a radio station it is not as easy as it can appear at first glance. Remember that this is a serious issue and that you cannot start without having planned everything. Usually, an LPFM station has a very low budget and for this reason some people may think to look around for the lowest prices on the equipment that they need. I understand that price is a big issue, but money has to be spent to prevent possible future failures that mean much more money then.
The following pertains to the RF part of a Radio Station beginning with the antenna, feeding cable, all the way down to the transmitter and protecting devices. In taking consideration of these points a station can avoid possible setbacks.
This is a very important part of the transmitting system. The choice needs to be made according to the area where it has to be installed. Stainless steel is always recommended especially where the weather is harsh or in close proximity of the ocean. Bad weather (ice and snow) can affect the performance of the antenna unless protected with a fiberglass radome. A poorly built antenna can also cause a failure in the transmitter creating a chain reaction.
Installing the antenna is something that needs to be done with the up most care. Once the antenna is mounted on its mast, you must understand that it has to face many possible weather conditions; for this reason spending more time in the installation will save you more money in the future. When rigid or semi-rigid cable is used for the feeding line, it is always advisable to connect it to the antenna through a 3 foot pigtail, made by flexible cable that will not transmit the vibrations to the antenna connector and that will act as a spring between the antenna connector and the rigid line.
It is very important to properly tape the connectors. They must be well protected so that water does not go in between the rubber protection and the copper shield. If this occurs it will cause a rapid deterioration of the cable with possible increase of the reflected power. Tape all connectors with vulcanized tape.
Grounding the antenna is another very important issue. Cable companies sell grounding kits for almost every type of cable. The best would be to ground the feeding line every 3-4 feet with these grounding devices. If this is not possible, then at least do the grounding every 10 feet. All of our antennas are grounded to avoid lightning and electrostatic problems.
The choice of the cable is crucial. Most of the time stations consider having 30-50 more watts of power from the transmitter. However, an incorrect choice of cable can cause a loss of a hundred or more watts. It is important for stations to understand how power can be gained and lost.
Half-inch cable should be the minimum requirement for an LPFM. It is still maneuverable and has a contained loss. Please avoid using low cost cable like RG8 or RG 213 as you will loose almost all of your transmitting power in this cable.
As previously mentioned, good grounding is an important requirement. But another very important tip is that the cable should never be tightly curved (since in that point the impedance could vary and the performance of the cable will be compromised).
Particular care should be taken when the cable enters the building. Make sure it curves adequately to enter the building and be careful not to smash the cable: do not bend sharply along the corners of a wall and make sure that it is fastened and out of the way, so that no one can inadvertently damage it when walking around the site.
Usually this is considered as the first item to purchase, but for me it is the last piece of the chain. For example, in many cases the FCC has granted 100 Watts ERP which means:
TX Power + Antenna Gain + Cable Loss + Other losses = 100W
When using a single bay antenna with a typical power gain of 0.47 and 100 feet of ½ inch cable, the choice ends up being a 250W transmitter.
Remember that the transmitter is an electronic device and it is subject to stress. When you buy a 250W transmitter and you run it at 250 watts, it means the same as if you run a fast car at its maximum speed 24 hours a day. You must be aware that there is a good chance that something could happen, especially if everything has not been installed properly. If we have a 500 W transmitter and we run it at 250W the product has more flexibility and less stress, like traveling in a comfortable car at 70 miles per hour.
The bad news is that a 500W transmitter is more expensive; this is true, though the additional outlay will more than pay for itself in the long run in terms of fewer problems and less repair expenses. Think about it!
Another solution could be using a 2 bay antenna; with this array, in the same situation, you will need only 150 watts for your 100W ERP. If for some reason a 500W transmitter is not in the plan, then it is a good idea, instead of buying a 150W transmitter, buying a 250W and running it at 150W maximum.
HOW TO PROTECT YOUR PRECIOUS TRANSMITTER
Let me say some words of advice about my favorite topic. During all of these years, I have tried very hard to convince people to spend money to protect their devices. In many cases, customers have listened to me and have called me very satisfied later, saying that during a storm, when many radio stations where off the air, theirs were still broadcasting.
I do not pretend to convince anybody, however I must bring to your attention a couple of things that prevent most of the breaks. Your transmitter can be assaulted from 2 different points; from the AC Line and from the antenna. How does one protect it from these attacks?
Most of the sites have a surge protector, but let me tell you that these are not enough at all; if you want to have the maximum isolation between the AC line and your transmitter, you have to install an Isolation transformer. This device is a regular transformer 1 to 1 (which means input 110V- output 110V) that physically isolates the main AC line from the input of your transmitter. This device can be found in used condition at around 300 dollars. This protection can save you much more money in the long run. In addition, these transformers usually have auxiliary outlets, so you can protect also small devices that can be on the site such as computers, receivers etc.
You can take the precautions as specified in the installation suggestions, but you will always be at risk that a lighting strike or another electrical charge may come down through the antenna and go straight into your transmitter. The final mosfet amplifier (this is the heart of the transmitter) is a very delicate and sensitive device. Every small electrical charge can damage it in an irreversible way. The best way to protect it from whatever current that is coming down through the antenna is to physically isolate the transmitter from the antenna. The device that can do this is called a Cavity Filter and it can be composed of one, two or three cells. We usually recommend a dual cell cavity filter. This device creates an isolation that prevents anything from reaching the transmitter. You can imagine it like a big fuse that absorbs everything. Plus it saves the transmitter in case of antenna failure. In fact, even if the antenna fails, the cavity will still provide the transmitter the 50 ohms impedance acting as a dummy load. Finally, the Cavity filter (as the word says) cleans the emission from all possible spurious components and avoids intermodulation products when more transmitters are in the same location. Again, the cavity filter is an additional cost but it can save you money in the future.
HOW TO MAINTAIN YOUR PRECIOUS TRANSMITTER
Your transmitting system is not something that you set up and then you forget ! You need to take care of it like if it was your car. Clean the transmitter, keep it in a clean place. (I got units with everything inside!). Keep replacement in stock, specially those parts that are subject to failure (fans, connectors etc.)
If you have no skill, then you MUST have a replacement unit handy ! Otherwise be prepared to stay off the air for some days.
This advice comes from 25 years of experience in the field. Through the years I have learned that these suggestions set the stage for a more efficient and smoother broadcasting experience instead of a sale gimmick. I hope these tips can help every new broadcaster in having the greatest satisfaction in their LPFM station.
The LPFM Store