I wrote this years ago, and after receiving several private messages recently I felt it was time to re-post it here.
Disclaimer: Cut me a bit of slack- engineers have submitted Doctoral Thesis' on any one of the following paragraphs. This is the Radio 101 summary.
Disclaimer: 2-Meter Amateur and 155 MHz Public safety are virtually identical except for the wavelength. The same rules apply, but with slightly different dimensions.
Antenna 101:
'Executive one-line summary":
Drill a 3/4" hole in the roof, install a NMO mount with quality RG58/U coax, and screw on a simple 1/4 wavelength commercial plain-wire antenna.
Rules of radio:
Rule #1: Antennas are one third science, one third black art and one third dumb luck.
Rule #2: The only difference between 'Amateur' and 'Professional' antenna systems is how they are installed and maintained. 'Should' work vs. 'Must' work. "I'll fix it next week" vs. "Go out to the cold windy hilltop on a dark and stormy night and fix it NOW!" "Bubba install" vs. "Exceeds R-56 spec." "Commercial and Professional" vs. discount off-shore.
Rule #3: Two mistakes may cancel each other out, resulting in an inexplicably functioning antenna system. If discovered and documented may result in receiving a PHD.
Rule #4: Build all antenna systems for transmitting, even if it is Rx-only. Rx is incredibly forgiving. Tx must be accurate, and if you get that right Rx in the same band will function just fine (with provisos).
Rule #5: A bad transmitting antenna system - including the coax - will reflect RF energy back into your transmitter final PA, turn into heat, and let the 'magic factory smoke' escape and drift away. Transmitting into a SWR of 2:1 means 100 BTU of heat is being returned to the back of your 60 watt radio. Transmitting into a RF short is radio suicide.
Rule #6: RF goes by its own quirky set of rules. A DC short does not necessarily mean a bad (or good) antenna system. A folded dipole base antenna is designed to function with one end of each dipole loop connected to ground.
Rule #7: Water will travel two feet uphill to find a pin-hole in your weather seals just because it can. Water is patient and insidious. Water carrying road salt is pure evil.
Rule #8: RF does not care how cool looking your antenna is, or how much you paid for it. RF loves simple, tuned resonance, clean and waterproof, and low-loss coax with properly installed connectors.
Rule #9: A simple open dipole antenna at the end of a hundred feet of quality, properly installed and weather sealed low loss coax attached to a $100 radio will out-perform an expensive 'exotic' antenna connected to a $10,000 radio by twenty feet of crap coax.
Rule #10: Radio end-users will always make antenna system selection, installation and maintenance far more complicated than it really is.
Theory Absolutes:
For any antenna to resonate at a transmitting frequency and send RF energy into the air it must be a variation of a 1/2 wavelength dipole (or a resonant multiple), or a phased array of resonant dipole elements.
And, the antenna must be of an electrical resonant length to provide about 50 ohms RF impedance - not DC resistance - to the radio. Coils in the radiator shorten the physical length while capacitance in (or near) the radiator lengthens it.
And, the antenna must be located where reflected energy from nearby elements/surfaces does not interfere with the resonance of the antenna, or where the reflection creates a desired directional pattern.
And, the antenna must be physically located in the desired energy path to/from the remote radio, and not blocked by vehicle or building parts.
Practical Use:
For mobile 2-Meter antennas, practical limitations mean that you have these choices:
1- A 1/4 wave whip (1/2 of the 1/2 wave dipole) with the feed-point attached to a conductive surface about 1/4 wavelength in -most- directions, allowing that conductive mounting surface to act as a ground plane and the other half of the 1/2 wave dipole. Electrically these are no different from a base station antenna consisting of a 1/4 wave vertical radiator and a series of horizontal elements forming the 'ground plane.'
For 2-Meter this would be a 18~19" vertical whip (17.75" for public safety) on a 3/4 hole NMO mount through a hole in the roof or trunk surface, This type is the standard for urban/metro public safety. Not because this is the easiest and cheapest: these antennas work very well across a wide band of frequencies (plus or minus 8 MHz) provided 0 antenna gain will produce the desired performance. "X" watts into the feed point will give an ERP of "X" watts. A -112 db received signal will be directed into the coax at -112 db.
1A- As above, but instead of a direct electrical connection to the roof, a magnetic 3-1/3" base inductively couples the feed point to the ground plane. Sometimes these work fine, but mag mounts are subject to moisture and many other issues causing inductive failure, any one of which will degrade performance, or let the magic smoke out.
2- A variation of #1 above, but using a 5/8 wavelength vertical radiator. This is a theoretical physical length of about 60". To reduce the physical length a coil is incorporated in the radiator, allowing for a practical physical length of less than 60" but maintaining the electrical resonance. This type of antenna results in a gain of about 3 db by 'stealing' or 'redirecting' RF energy from high and low angles and redirecting it to/from the horizon. This DOUBLES both the Tx and Rx effective gain. "X" watts into the feed point will give an ERP of about "X-times-two" watts towards the horizon, with less at higher angles. A -112 db received signal coming from the horizon will be directed into the coax at -109 db, doubling the amount of RF energy going to the receiver. This is the standard for rural public safety.
NOTE: Long antennas on mag mounts have the same issues as above, plus a tendency to fall off and break glass.
3- A 1/2 wavelength vertical radiator. This wavelength radiator has the happy coincidence of being able to resonate all by itself without the 'other half' of the dipole underneath, plus gives you about 2 db of gain at the resonant frequency. They have the disadvantage of only being able to resonate within a narrow bandwidth, sometimes less than 1 MHz. If tuned for 146 MHz, best of luck at 144 or 148. This is the standard for plastic/fiberglass roof tractors and ambulances, and must ALWAYS be tuned for the TX frequency.
Anything else is simply a variation on the above, sometimes without any improvement of performance.
Mobile Mounts:
Dandy. You have decided on an antenna. Now, how do you mount it to your car/truck/boat/tractor/lawnmover without your XYL throwing a fit?
a- Best: Bite the bullet, drill a 0.750000" hole in the roof, and install a commercial quality NMO mount. Most NMO's are designed to be installed 'blind', meaning you do not have to drop the headliner. As long as you can fish the coax to one side, you are golden. If all the o-rings and silicon grease are installed properly, it will NEVER leak. If it does, there is a 99% chance the o-ring moved, got cut or is missing.
CAUTION: Measure twice, drill once. Do not drill through a frame or stiffener, or drill into a cavity from which there is no exit. Do not drill too deep and go through the headliner, or into a roof light assembly. Yank down 'just enough' of the door seal strip to get from the side to the front 'A' column. If it isn't coming down, pull harder. It will. Route the coax behind airbags so the airbag can explode without being wrapped up in the coax.
HINT: 1/4" ice maker water supply tube- the white-ish stiff kind - works perfect for fishing RG58 coax. Strip off 3"of the outer jacket and insert the coax braid into the tube. Push into the 3/4" hole and work it to the door. Leave the tube on the end of the coax until it is routed all the way to the radio.
HINT: check the coax for nicks and scrapes without letting it leave your hand, lest you discover after routing that the coax is wrapped around the door seal strip or snuck through the steering wheel.
NOTE: for most pickup trucks, pull the third brake light assembly from over the back glass. There is just enough room in the cavity to drill the hole, and it is fairly easy to fish the coax to one side. No, the location is not theoretically perfect, but it will work just fine. Trust me. When you trade the vehicle, leave the mount and coax for the next guy.
XYL Note: This is the most discrete option, and should go un-noticed after a week. NMO mounts are universal, meaning you can exchange the 1/4 wavelength whip for a 5/8 wavelength base loaded coil for "vacation travel." Remembering to swap them back afterwards is at your option.
b- Magnetic mounts. Sometimes they work, sometimes they don't. All I can say is good luck. Unless the coax is well protected figure on getting a new one every two years or 500 door slams, whichever comes first. Your mileage may vary.
XYL Note: A mag mount will soon be forgotten - until it is swept off the roof and crashes through the glass at the car wash.
c- Glass mounts. If all the stars align, they may work okay, but with at least a 1.5 db installation loss for VHF. Bandwidth sucks. The metallic content of tinted glass can ruin your day. The radiator will always be partially shielded by the vehicle. Zero (unity) gain less mounting loss db is the best you can hope for.
NOTE: the capacitive coupling for UHF and 900 MHz cellular is far different, meaning that glass-mounts may be okay for those applications. That DOES NOT cross into lower 2-meter VHF bands. Avoid if possible, install and tune temporarily, and keep your receipt.
d- Raised feed point mounts. Break out the welder or go online and bend over. By design, these types of antenna mounts - bumper or trailer hitch mounts included - incorporate all of the disadvantages of the worst RF nightmares with all of the mounting challenges and physical damage issues Mr. Murphy can think of. They also scream "CAUTION: Radio geek" and "ATTTENTION : Good stuff to steal inside."
6-Meter, 10-Meter and HF antennas have different requirements than 2-Meter and up. Different rules apply to 2-Meter. Much like spark plugs and rectal thermometers, they are both useful but not interchangeable.
XYL Note: "WHAT is THAT?!?"
KISS:
Many vendors market antennas looking like their "technology" was reverse-engineered from UFO's. The CB and TV markets are full of them. 99% are marketing glitz, with most of the external gizmos ineffective or not even functional.
In the commercial and public safety worlds where failure is not an option and equipment must always work, you will never see a X-files design. There are only four kinds of VHF or UHF antennas:
Mobile installs consisting of a 1/4 wavelength (or resonant factor wavelength) whip (with or without loading coils) on a permanent NMO hole mount. Loading coils may be open, like the common 800 MHz or cellular antennas, or enclosed in the base or midpoint.
Exception: Small domes or specialty low-profile antennas appear only where absolutely necessary, such as on tall fire apparatus or transit buses, and are used only in locations where base infrastructure is exceptional.
Base antennas with 1, 2, 4, 8 or 16 exposed folded dipoles, connected by a phasing harness.
Base antennas with a fiberglass tube containing a long ugly sequence of coax soldered to turn 'inside out' and reverse shield to core every 1/4 or 1/2 wavelength. Boat antennas are the same thing, but resonate at 156 MHz +/- 5 MHz. Cheap glass radome antennas rattle because the coax array inside is free to move. Better ones don't rattle.
Base antennas with a single 1/4 wavelength radiator above a series of horizontal 1/4 wavelength radiators extending from the feed point to create a ground plane.
Amateur and other exceptions:
J-Poles. These are often home made, and with a little tweaking work quite well. The theory is a matter for another day. Normally a base station antenna, but theoretically possible for mobile use as well. Do your homework before braking out the soldering torch.
Discones: These multiple radiator element (usually 16) base antennas work amazingly well both Rx AND Tx, and resonate across a wide range of bands. Wind loading is a problem, so they are best attached to a heavy mast. Not normally applicable in mobile environments unless you drive a battleship to work.
Directional Yagi, plate and reflector directional antennas: a science all to themselves.
Mobile dome, low profile and specialty 'hidden' antennas: For UHF and up, these are mostly 1/4 wavelength radiators, with or without loading coils, often contained in a dome. They work fine most of the time. For 2-Meter, they are barely okay at a SINGLE tuned frequency. They suck more than 0.25 MHz off of their tuned resonance. Tuned to the input of the only 2-meter repeater you will ever work they are okay, provided the repeater down-link RF output 600 KHz away is strong enough. Simplex Tx will probably fail.
2-Meter and 70-CM: The physical wavelengths of these amateur bands are a harmonic multiple by plan, NOT by coincidence. This does not apply to public safety bands, where 155 MHz VHF and 455 MHz UHF common frequencies are not harmonic wavelengths.
A final note.
A substantial income stream for me comes from designing, installing and maintaining public safety mobile and base station antenna systems. Please note that I like to use the term 'antenna systems." 50% of performance comes from the coax, and another 25% from the antenna mounting location. What is "up there" is often less important.
I have seen many amateurs agonize for months over the printed specifications of one antenna vs. another because of a written claim of 3.0 db gain vs. 2.7925 db. Then they throw the whole discussion out the window by installing their antenna on a questionable mounting or use crap coax.
The reality is very simple: Unless you are addicted to working hyper-weak -124 db contacts, go with simple, clean tight antennas on RG58/U double shield coax. Get your Tx RF energy efficiently into the air with minimal loss. Most of us listen to greater than -80 db signals anyway; so you will not notice the difference between a -115 db received signal and a -114.5 db signal. If you can, this discussion is far too simple for you.
Disclaimer: Cut me a bit of slack- engineers have submitted Doctoral Thesis' on any one of the following paragraphs. This is the Radio 101 summary.
Disclaimer: 2-Meter Amateur and 155 MHz Public safety are virtually identical except for the wavelength. The same rules apply, but with slightly different dimensions.
Antenna 101:
'Executive one-line summary":
Drill a 3/4" hole in the roof, install a NMO mount with quality RG58/U coax, and screw on a simple 1/4 wavelength commercial plain-wire antenna.
Rules of radio:
Rule #1: Antennas are one third science, one third black art and one third dumb luck.
Rule #2: The only difference between 'Amateur' and 'Professional' antenna systems is how they are installed and maintained. 'Should' work vs. 'Must' work. "I'll fix it next week" vs. "Go out to the cold windy hilltop on a dark and stormy night and fix it NOW!" "Bubba install" vs. "Exceeds R-56 spec." "Commercial and Professional" vs. discount off-shore.
Rule #3: Two mistakes may cancel each other out, resulting in an inexplicably functioning antenna system. If discovered and documented may result in receiving a PHD.
Rule #4: Build all antenna systems for transmitting, even if it is Rx-only. Rx is incredibly forgiving. Tx must be accurate, and if you get that right Rx in the same band will function just fine (with provisos).
Rule #5: A bad transmitting antenna system - including the coax - will reflect RF energy back into your transmitter final PA, turn into heat, and let the 'magic factory smoke' escape and drift away. Transmitting into a SWR of 2:1 means 100 BTU of heat is being returned to the back of your 60 watt radio. Transmitting into a RF short is radio suicide.
Rule #6: RF goes by its own quirky set of rules. A DC short does not necessarily mean a bad (or good) antenna system. A folded dipole base antenna is designed to function with one end of each dipole loop connected to ground.
Rule #7: Water will travel two feet uphill to find a pin-hole in your weather seals just because it can. Water is patient and insidious. Water carrying road salt is pure evil.
Rule #8: RF does not care how cool looking your antenna is, or how much you paid for it. RF loves simple, tuned resonance, clean and waterproof, and low-loss coax with properly installed connectors.
Rule #9: A simple open dipole antenna at the end of a hundred feet of quality, properly installed and weather sealed low loss coax attached to a $100 radio will out-perform an expensive 'exotic' antenna connected to a $10,000 radio by twenty feet of crap coax.
Rule #10: Radio end-users will always make antenna system selection, installation and maintenance far more complicated than it really is.
Theory Absolutes:
For any antenna to resonate at a transmitting frequency and send RF energy into the air it must be a variation of a 1/2 wavelength dipole (or a resonant multiple), or a phased array of resonant dipole elements.
And, the antenna must be of an electrical resonant length to provide about 50 ohms RF impedance - not DC resistance - to the radio. Coils in the radiator shorten the physical length while capacitance in (or near) the radiator lengthens it.
And, the antenna must be located where reflected energy from nearby elements/surfaces does not interfere with the resonance of the antenna, or where the reflection creates a desired directional pattern.
And, the antenna must be physically located in the desired energy path to/from the remote radio, and not blocked by vehicle or building parts.
Practical Use:
For mobile 2-Meter antennas, practical limitations mean that you have these choices:
1- A 1/4 wave whip (1/2 of the 1/2 wave dipole) with the feed-point attached to a conductive surface about 1/4 wavelength in -most- directions, allowing that conductive mounting surface to act as a ground plane and the other half of the 1/2 wave dipole. Electrically these are no different from a base station antenna consisting of a 1/4 wave vertical radiator and a series of horizontal elements forming the 'ground plane.'
For 2-Meter this would be a 18~19" vertical whip (17.75" for public safety) on a 3/4 hole NMO mount through a hole in the roof or trunk surface, This type is the standard for urban/metro public safety. Not because this is the easiest and cheapest: these antennas work very well across a wide band of frequencies (plus or minus 8 MHz) provided 0 antenna gain will produce the desired performance. "X" watts into the feed point will give an ERP of "X" watts. A -112 db received signal will be directed into the coax at -112 db.
1A- As above, but instead of a direct electrical connection to the roof, a magnetic 3-1/3" base inductively couples the feed point to the ground plane. Sometimes these work fine, but mag mounts are subject to moisture and many other issues causing inductive failure, any one of which will degrade performance, or let the magic smoke out.
2- A variation of #1 above, but using a 5/8 wavelength vertical radiator. This is a theoretical physical length of about 60". To reduce the physical length a coil is incorporated in the radiator, allowing for a practical physical length of less than 60" but maintaining the electrical resonance. This type of antenna results in a gain of about 3 db by 'stealing' or 'redirecting' RF energy from high and low angles and redirecting it to/from the horizon. This DOUBLES both the Tx and Rx effective gain. "X" watts into the feed point will give an ERP of about "X-times-two" watts towards the horizon, with less at higher angles. A -112 db received signal coming from the horizon will be directed into the coax at -109 db, doubling the amount of RF energy going to the receiver. This is the standard for rural public safety.
NOTE: Long antennas on mag mounts have the same issues as above, plus a tendency to fall off and break glass.
3- A 1/2 wavelength vertical radiator. This wavelength radiator has the happy coincidence of being able to resonate all by itself without the 'other half' of the dipole underneath, plus gives you about 2 db of gain at the resonant frequency. They have the disadvantage of only being able to resonate within a narrow bandwidth, sometimes less than 1 MHz. If tuned for 146 MHz, best of luck at 144 or 148. This is the standard for plastic/fiberglass roof tractors and ambulances, and must ALWAYS be tuned for the TX frequency.
Anything else is simply a variation on the above, sometimes without any improvement of performance.
Mobile Mounts:
Dandy. You have decided on an antenna. Now, how do you mount it to your car/truck/boat/tractor/lawnmover without your XYL throwing a fit?
a- Best: Bite the bullet, drill a 0.750000" hole in the roof, and install a commercial quality NMO mount. Most NMO's are designed to be installed 'blind', meaning you do not have to drop the headliner. As long as you can fish the coax to one side, you are golden. If all the o-rings and silicon grease are installed properly, it will NEVER leak. If it does, there is a 99% chance the o-ring moved, got cut or is missing.
CAUTION: Measure twice, drill once. Do not drill through a frame or stiffener, or drill into a cavity from which there is no exit. Do not drill too deep and go through the headliner, or into a roof light assembly. Yank down 'just enough' of the door seal strip to get from the side to the front 'A' column. If it isn't coming down, pull harder. It will. Route the coax behind airbags so the airbag can explode without being wrapped up in the coax.
HINT: 1/4" ice maker water supply tube- the white-ish stiff kind - works perfect for fishing RG58 coax. Strip off 3"of the outer jacket and insert the coax braid into the tube. Push into the 3/4" hole and work it to the door. Leave the tube on the end of the coax until it is routed all the way to the radio.
HINT: check the coax for nicks and scrapes without letting it leave your hand, lest you discover after routing that the coax is wrapped around the door seal strip or snuck through the steering wheel.
NOTE: for most pickup trucks, pull the third brake light assembly from over the back glass. There is just enough room in the cavity to drill the hole, and it is fairly easy to fish the coax to one side. No, the location is not theoretically perfect, but it will work just fine. Trust me. When you trade the vehicle, leave the mount and coax for the next guy.
XYL Note: This is the most discrete option, and should go un-noticed after a week. NMO mounts are universal, meaning you can exchange the 1/4 wavelength whip for a 5/8 wavelength base loaded coil for "vacation travel." Remembering to swap them back afterwards is at your option.
b- Magnetic mounts. Sometimes they work, sometimes they don't. All I can say is good luck. Unless the coax is well protected figure on getting a new one every two years or 500 door slams, whichever comes first. Your mileage may vary.
XYL Note: A mag mount will soon be forgotten - until it is swept off the roof and crashes through the glass at the car wash.
c- Glass mounts. If all the stars align, they may work okay, but with at least a 1.5 db installation loss for VHF. Bandwidth sucks. The metallic content of tinted glass can ruin your day. The radiator will always be partially shielded by the vehicle. Zero (unity) gain less mounting loss db is the best you can hope for.
NOTE: the capacitive coupling for UHF and 900 MHz cellular is far different, meaning that glass-mounts may be okay for those applications. That DOES NOT cross into lower 2-meter VHF bands. Avoid if possible, install and tune temporarily, and keep your receipt.
d- Raised feed point mounts. Break out the welder or go online and bend over. By design, these types of antenna mounts - bumper or trailer hitch mounts included - incorporate all of the disadvantages of the worst RF nightmares with all of the mounting challenges and physical damage issues Mr. Murphy can think of. They also scream "CAUTION: Radio geek" and "ATTTENTION : Good stuff to steal inside."
6-Meter, 10-Meter and HF antennas have different requirements than 2-Meter and up. Different rules apply to 2-Meter. Much like spark plugs and rectal thermometers, they are both useful but not interchangeable.
XYL Note: "WHAT is THAT?!?"
KISS:
Many vendors market antennas looking like their "technology" was reverse-engineered from UFO's. The CB and TV markets are full of them. 99% are marketing glitz, with most of the external gizmos ineffective or not even functional.
In the commercial and public safety worlds where failure is not an option and equipment must always work, you will never see a X-files design. There are only four kinds of VHF or UHF antennas:
Mobile installs consisting of a 1/4 wavelength (or resonant factor wavelength) whip (with or without loading coils) on a permanent NMO hole mount. Loading coils may be open, like the common 800 MHz or cellular antennas, or enclosed in the base or midpoint.
Exception: Small domes or specialty low-profile antennas appear only where absolutely necessary, such as on tall fire apparatus or transit buses, and are used only in locations where base infrastructure is exceptional.
Base antennas with 1, 2, 4, 8 or 16 exposed folded dipoles, connected by a phasing harness.
Base antennas with a fiberglass tube containing a long ugly sequence of coax soldered to turn 'inside out' and reverse shield to core every 1/4 or 1/2 wavelength. Boat antennas are the same thing, but resonate at 156 MHz +/- 5 MHz. Cheap glass radome antennas rattle because the coax array inside is free to move. Better ones don't rattle.
Base antennas with a single 1/4 wavelength radiator above a series of horizontal 1/4 wavelength radiators extending from the feed point to create a ground plane.
Amateur and other exceptions:
J-Poles. These are often home made, and with a little tweaking work quite well. The theory is a matter for another day. Normally a base station antenna, but theoretically possible for mobile use as well. Do your homework before braking out the soldering torch.
Discones: These multiple radiator element (usually 16) base antennas work amazingly well both Rx AND Tx, and resonate across a wide range of bands. Wind loading is a problem, so they are best attached to a heavy mast. Not normally applicable in mobile environments unless you drive a battleship to work.
Directional Yagi, plate and reflector directional antennas: a science all to themselves.
Mobile dome, low profile and specialty 'hidden' antennas: For UHF and up, these are mostly 1/4 wavelength radiators, with or without loading coils, often contained in a dome. They work fine most of the time. For 2-Meter, they are barely okay at a SINGLE tuned frequency. They suck more than 0.25 MHz off of their tuned resonance. Tuned to the input of the only 2-meter repeater you will ever work they are okay, provided the repeater down-link RF output 600 KHz away is strong enough. Simplex Tx will probably fail.
2-Meter and 70-CM: The physical wavelengths of these amateur bands are a harmonic multiple by plan, NOT by coincidence. This does not apply to public safety bands, where 155 MHz VHF and 455 MHz UHF common frequencies are not harmonic wavelengths.
A final note.
A substantial income stream for me comes from designing, installing and maintaining public safety mobile and base station antenna systems. Please note that I like to use the term 'antenna systems." 50% of performance comes from the coax, and another 25% from the antenna mounting location. What is "up there" is often less important.
I have seen many amateurs agonize for months over the printed specifications of one antenna vs. another because of a written claim of 3.0 db gain vs. 2.7925 db. Then they throw the whole discussion out the window by installing their antenna on a questionable mounting or use crap coax.
The reality is very simple: Unless you are addicted to working hyper-weak -124 db contacts, go with simple, clean tight antennas on RG58/U double shield coax. Get your Tx RF energy efficiently into the air with minimal loss. Most of us listen to greater than -80 db signals anyway; so you will not notice the difference between a -115 db received signal and a -114.5 db signal. If you can, this discussion is far too simple for you.
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