2m Antenna Questions

[4x4mike]

I am completely new to HAM. I've bought a book and I'm studying and I have a 2800m in the mail right now. My question is regarding antennas.

I have a full bodied 4runner and would like to put an antenna on the roof. Magnet mount would be nice but I have no experience with good models, brands, etc. I do park in parking garages so it would be nice if the whip can be removed, does this exist?

Another question is antenna length. This is something else I know zero about. I used to have a huge whip for my cb and have since gone to a small lil wil whip. I see many different antennas for HAM (from a few inches in length to 120" whips). What can I get away with. I have a single band 2m radio. Can I go short? Like a CB will that effect it's range?

I currently have a magnet mount Wilson Lil Wil on the roof towards the rear of the vehicle for my cb and would like to keep that there. Can I mount the HAM antenna on the roof with the cb antenna up there? Or will that cause interference? If I can have them both up there how far apart should they be. Reason for asking is that space is somewhat limited due to a cargo rack.

Another option is mounted to the fender by the hood. The mounts online are quite expensive for what they are so I was thinking about making something. This will put the antenna a little lower and farther away from the CB antenna. Will a little homemade bracket give enough ground plane? Will the lower location effect range?


Thank you for your time. Expo is a great place with many knowledgeable folks so I was hoping for a little aid. I know these are simple questions for those in the know. Hopefully I'll get rolling soon and have tougher questions for you all.

Mike

 

[PaulXyZ]

I am completely new to HAM. I've bought a book and I'm studying and I have a 2800m in the mail right now. My question is regarding antennas.

I have a full bodied 4runner and would like to put an antenna on the roof. Magnet mount would be nice but I have no experience with good models, brands, etc. I do park in parking garages so it would be nice if the whip can be removed, does this exist?

Another question is antenna length. This is something else I know zero about. I used to have a huge whip for my cb and have since gone to a small lil wil whip. I see many different antennas for HAM (from a few inches in length to 120" whips). What can I get away with. I have a single band 2m radio. Can I go short? Like a CB will that effect it's range?

I currently have a magnet mount Wilson Lil Wil on the roof towards the rear of the vehicle for my cb and would like to keep that there. Can I mount the HAM antenna on the roof with the cb antenna up there? Or will that cause interference? If I can have them both up there how far apart should they be. Reason for asking is that space is somewhat limited due to a cargo rack.

Another option is mounted to the fender by the hood. The mounts online are quite expensive for what they are so I was thinking about making something. This will put the antenna a little lower and farther away from the CB antenna. Will a little homemade bracket give enough ground plane? Will the lower location effect range?


Thank you for your time. Expo is a great place with many knowledgeable folks so I was hoping for a little aid. I know these are simple questions for those in the know. Hopefully I'll get rolling soon and have tougher questions for you all.

Mike

Hi, I'm an amateur extra class and know a wee bit about antennas.

The best location for a 2M antenna is in the center of the roof - drill a hole or magnetic mount work well. On my Xterra I yielded the center of the roof for the side making it easier to get to and further from the trees on the side of the roads.

Size is gain. Depending on where you live and how far to the repeaters you'll need more or less gain. Being the comm geek I have various size antennas - I have a little one for around town where I can hit the repeater on the mountain above me with 1W of power and a big one for off-road where I need to hit distant repeaters. The little antenna is 16" tall and has 1.5 dB gain, the big one is 55" tall and has 4.5 dB gain.

Antenna gain is "free" power.

If I put 50W into the 1.5 dB gain antenna I get 70W off of my antenna. Nice 20W for free. Run that same 50W into a 4.5 dB antenna I get about 140 watts of power. Very nice.

Antennas ought to be mounted as far as part as possible. Since CB is in the 11 meter band and 2 meter is in the well, 2M band they are pretty far apart and won't couple together too badly. Still practice is to keep them apart.

Antennas come with various bases that mate with various mounts. There are three common ones - NMO. 3/8th Stud, and UHF (or PO mount). I like the NMO but just make sure you get the right antenna for your mount or via-versa.

 

[4x4mike]

Thanks for the reply.

Do you have a suggestion for a through roof mount? One that can accept different length antennas? I like your idea. I can carry all kinds of different lengths with ease and like the idea of "free power" to really reach out there. I have to do some research on the repeaters in my area and where I wheel. I'm pretty sure I can find a spot on my roof at least 5 feet from my cb antenna to drill and hole. Key would be a base that can accept the different length antennas. Can you also offer some suggestions on the different antennas or what you use (I'm assuming you've got them in the same base)?

Again thanks.

 

[Paul 3]

4x4mike,
among the many great resources for locating repeaters... http://k5ehx.net/repeaters/qrepeater.php just click on the map and you will get plenty of listings. Out there in California you've got more repeaters than you can count. You should have no problem finding several Amateur Radio Clubs out there that can point you in the right direction as well.

Paul

 

[DaveInDenver]

like the idea of "free power" to really reach out there

There is nothing that is free about this. A gain antenna is not making additional power, the energy you send up the coax is all the energy that radiated, minus whatever is lost to heat, ground absorption or mismatch. The general misunderstanding here is that you are just changing the radiation pattern. So any energy you focus through gain is lost someplace else. Don't get into the biggest gain number until you realize that you might or might NOT want gain depending on what you are trying to do.

With a vertical monopole antenna the pattern is created in elevation orthogonal to the radiator and in azimuth is parallel. This also depends heavily on the configuration and quality of your ground. When most VHF antennas are shown to have gain it is that they sacrifice elevation energy for azimuthal and for line of sight this is generally preferred. But that also means the antenna will not create as much energy up as the angle off the horizon increases. For talking to your buddy across a lake or the sand flats, that is good. For talking to someone above you in a slot canyon that is bad. Remember to extrapolate the angle because it is possible that even when in the open you have to consider that the repeater or other contact is physically higher than you at great distance.

Also remember that a primary principle of antenna is reciprocity. So its radiating pattern is going to be similar to its receptive pattern. A high gain antenna can have significant areas where it is deaf.

Look at this chart which shows very basically a comparison between 1/4, 1/2 and 5/8 wavelength monopole antennas. The envelope of the energy is not going to be much different for each, but where the energy goes does. So when someone is referring to 'gain' they usually mean in azimuth, i.e. contact range in the horizon with a vertically orientated monopole. But you could legitimately say that a 1/4 wavelength vertical shows gains over a 5/8 wavelength in elevations over 45 degrees off the horizon.

Think of all this remembering that RF has the same properties as any other wave, light or sound. For example a gain antenna is doing to RF what a magnifying glass does for light. Yes, you can make something brighter or larger with one but you give up the larger picture the more you focus it. A yagi has tremendous forward gain (and reception) but is just about completely deaf to its back. This is similar to putting a cone over your ear. Yup, some things will be a lot louder but others you just won't hear anymore.

So let's say you are trying to stay in contact with two contacts, one at your elevation and another overhead. This might be in a S&R scenario with a helicopter and a missing person. The 1/4 will have the best chance of being non-selective in where your signal goes and your radio receives, while a 5/8 will not favor the helo. How that matters is neither always good nor bad, just be aware of the implications of different radiation patterns.

Or another example, you driving down the road with your whip flopping around like they naturally do. The 1/4 is going to be relatively immune to pattern induced fading, although the absolute range of your contact will go down. While OTOH the higher gain monopoles will let you touch out farther but the contact at the fringe will go in and out more if either or both antennas are not mechanically stable. You will hear this a lot on repeaters, where a station will alternate between hitting and not hitting a repeater as they are driving. That is probably their antenna waving around and alternating between the energy being focused towards the repeater and not even if they are located relatively close.

 

[cnynrat]

Mike -

I've adopted a similar strategy of having two different antennas for different situations. I went this way when I found that my taller antenna was really taking a beating on tree lined trails. I don't claim this is necessarily the best antenna setup, but they work well for me. If you want to go taller, or give up dual band compatibility I think you can get a little more gain. Here's what I use:

Comet CSB-7500 (42" long, 3.6 dB gain) - I use this antenna when I need to get out there and have the clearance to use a taller antenna. Comet has replaced this model with the CSB-750A.

Comet SBB-1 (16.5", flexible, 1.5 dB gain). I use the SBB -1 when I'm in a situation where I have low clearance, such as trails with lots of trees.

Both have a PL-259 (aka UHF) mount, so they are compatible with a wide variety of antenna mounts. I have mine on a lip mount on the back door of my FJ Cruiser. Not the best choice for antenna performance, but I wanted to preserve the ability to load stuff on my roof rack which takes away the center of the roof option.

 

[DaveInDenver]

BTW, I run a Larsen NMO2/70 with an open coil right in the middle of my truck roof. I would suggest to you to try a Larsen single band VHF (NMO150) or similar first. They are fairly cheap, very well made and a good balance pattern. Going much higher in gain seems for me to be too much compromise but I live in a place that elevation does factor in. I run a stubby, middle loaded 1/4 wavelength with a spring in places with low branches and that works very well in town, but the difference in range is noticeable when not using the repeaters.

 

[4x4mike]

Thank you for all of the insight and suggestions, this is exactly what I wanted. I don't have my license yet but this is part of my to do list for getting dialed in. I will look up these antenna suggestions tonight when I have a little time. Looks like the NMO150 is a popular choice.
Question about it though. Is it just a little nub on the roof? I'm guessing there is an allen key holding the antenna (kind of like my CB whip). This would be nice because it's easy on and off. Does the actual whip antenna need to be tuned like a CB? I'd probably just stash the unused whip under a seat or the carpet but would need to make sure it's tuned when I'm ready to use it. The product descriptions don't say much about these 2 questions.

 

[silvergrand]

Dave in Denver,

Thanks for presenting the RF chart in a way that makes sense to me! I'm new to the HAM world but have found it to be very enjoyable. The RF charts have always been hard for my little brain to understand, but no more!

Thanks,

KC9RWO

 

[ntsqd]

Thank you for all of the insight and suggestions, this is exactly what I wanted. I don't have my license yet but this is part of my to do list for getting dialed in. I will look up these antenna suggestions tonight when I have a little time. Looks like the NMO150 is a popular choice.
Question about it though. Is it just a little nub on the roof? I'm guessing there is an allen key holding the antenna (kind of like my CB whip). This would be nice because it's easy on and off. Does the actual whip antenna need to be tuned like a CB? I'd probably just stash the unused whip under a seat or the carpet but would need to make sure it's tuned when I'm ready to use it. The product descriptions don't say much about these 2 questions.

IME with Larsen antennas they do use a SHSS to pinch the whip into the base at the desired tuning point. Not a big deal though, the whole unit easily unscrews from the NMO mount. I have heard of, but never seen a cap that can be put in place over the NMO base. I use a little 16-ish inch whip instead. Which, ironically, is bent from an encounter with a branch.

Watch, now that I've posted that bit about the cap someone will post a link to such, and at a vendor that I frequent - meaning that I've been blind to it....

BTW, if you have one, your cb swr meter won't work.

 

[DaveInDenver]

Watch, now that I've posted that bit about the cap someone will post a link to such, and at a vendor that I frequent - meaning that I've been blind to it....

Larsen calls them 'NMO-CAP-B' in their catalog. Should be about $5 from any radio shop. And yes, you just spin the whole thing off the NMO mount, don't mess with the whip tuning set screw. You do that once and leave it.

 

[xtatik]

There is nothing that is free about this. A gain antenna is not making additional power, the energy you send up the coax is all the energy that radiated, minus whatever is lost to heat, ground absorption or mismatch. The general misunderstanding here is that you are just changing the radiation pattern. So any energy you focus through gain is lost someplace else. Don't get into the biggest gain number until you realize that you might or might NOT want gain depending on what you are trying to do.

With a vertical monopole antenna the pattern is created in elevation orthogonal to the radiator and in azimuth is parallel. This also depends heavily on the configuration and quality of your ground. When most VHF antennas are shown to have gain it is that they sacrifice elevation energy for azimuthal and for line of sight this is generally preferred. But that also means the antenna will not create as much energy up as the angle off the horizon increases. For talking to your buddy across a lake or the sand flats, that is good. For talking to someone above you in a slot canyon that is bad. Remember to extrapolate the angle because it is possible that even when in the open you have to consider that the repeater or other contact is physically higher than you at great distance.

Also remember that a primary principle of antenna is reciprocity. So its radiating pattern is going to be similar to its receptive pattern. A high gain antenna can have significant areas where it is deaf.

Look at this chart which shows very basically a comparison between 1/4, 1/2 and 5/8 wavelength monopole antennas. The envelope of the energy is not going to be much different for each, but where the energy goes does. So when someone is referring to 'gain' they usually mean in azimuth, i.e. contact range in the horizon with a vertically orientated monopole. But you could legitimately say that a 1/4 wavelength vertical shows gains over a 5/8 wavelength in elevations over 45 degrees off the horizon.

Think of all this remembering that RF has the same properties as any other wave, light or sound. For example a gain antenna is doing to RF what a magnifying glass does for light. Yes, you can make something brighter or larger with one but you give up the larger picture the more you focus it. A yagi has tremendous forward gain (and reception) but is just about completely deaf to its back. This is similar to putting a cone over your ear. Yup, some things will be a lot louder but others you just won't hear anymore.

So let's say you are trying to stay in contact with two contacts, one at your elevation and another overhead. This might be in a S&R scenario with a helicopter and a missing person. The 1/4 will have the best chance of being non-selective in where your signal goes and your radio receives, while a 5/8 will not favor the helo. How that matters is neither always good nor bad, just be aware of the implications of different radiation patterns.

Or another example, you driving down the road with your whip flopping around like they naturally do. The 1/4 is going to be relatively immune to pattern induced fading, although the absolute range of your contact will go down. While OTOH the higher gain monopoles will let you touch out farther but the contact at the fringe will go in and out more if either or both antennas are not mechanically stable. You will hear this a lot on repeaters, where a station will alternate between hitting and not hitting a repeater as they are driving. That is probably their antenna waving around and alternating between the energy being focused towards the repeater and not even if they are located relatively close.

Good stuff. It's also worth mentioning that nearly all antenna mfr'r's liberally state their gain numbers when marketing their products. They are software modeled, and the testing is based on an absolutely perfect condition for each individual antenna. These conditions are seldom anything like the actual conditions encountered while being operated by the consumer.

 

[DaveInDenver]

Good stuff. It's also worth mentioning that nearly all antenna mfr'r's liberally state their gain numbers when marketing their products. They are software modeled, and the testing is based on an absolutely perfect condition for each individual antenna. These conditions are seldom anything like the actual conditions encountered while being operated by the consumer.

What he is referring to is gain stated against dBi and dBd. dBi stands for decibel isotropic and dBd stands for decibel dipole. An isotropic antenna is a non-realizable point antenna that has a perfectly spherical pattern, literally a big perfect ball with a point antenna inside. An antenna must have a certain physical length to be resonant and so this obviously is only mathematically an antenna that we engineers and physics majors talked about in EM Fields II but doesn't exist in the real world.

So the next most ideal antenna is a Hertzian dipole, the classical 1/2 wavelength dipole that we've all seen and is actually possible to make. This has a non-uniform pattern but is still extremely symmetrical, looks like a doughnut with a very small volume of reduced radiation. Due to it's pattern it shows 2.2dB of gain over the isotropic ideal. So /all/ real antennas already have gain and directionality. But to make their numbers look bigger some (really most) manufacturers market their gain in dBi. So you have to look carefully if you are considering something unique beyond standard configurations.

Bottom line, just be aware of the conversion (dBi - 2.2dB = dBd) so that you can compare two antennas that might have their specs in different units.

And you are right, the manufacturer probably gives you theoretical gains from modeling software. But they do test them on an antenna range almost definitely. In reality a lot of things will affect your results, mostly the quality of your feedline, the ground plane, the surrounding ground (like your truck), etc. But OTOH the antenna is going to really perform roughly like they say and it's more a matter of how much you give up using junk coax and Rat Shack PL259 connectors. If there's 'free' energy to be had it's fixing up installations if ask me, a lot of people are giving away RF energy just heating the breaking down dielectric in their old coax with half a dozen sharp bends than they could ever get with a higher gain antenna.

 

[xtatik]

What he is referring to is gain stated against dBi and dBd. dBi stands for decibel isotropic and dBd stands for decibel dipole. An isotropic antenna is a non-realizable point antenna that has a perfectly spherical pattern, literally a big perfect ball with a point antenna inside. An antenna must have a certain physical length to be resonant and so this obviously is only mathematically an antenna that we engineers and physics majors talked about in EM Fields II but doesn't exist in the real world.

So the next most ideal antenna is a Hertzian dipole, the classical 1/2 wavelength dipole that we've all seen and is actually possible to make. This has a non-uniform pattern but is still extremely symmetrical, looks like a doughnut with a very small volume of reduced radiation. Due to it's pattern it shows 2.2dB of gain over the isotropic ideal. So /all/ real antennas already have gain and directionality. But to make their numbers look bigger some (really most) manufacturers market their gain in dBi. So you have to look carefully if you are considering something unique beyond standard configurations.

Bottom line, just be aware of the conversion (dBi - 2.2dB = dBd) so that you can compare two antennas that might have their specs in different units.

And you are right, the manufacturer probably gives you theoretical gains from modeling software. But they do test them on an antenna range almost definitely. In reality a lot of things will affect your results, mostly the quality of your feedline, the ground plane, the surrounding ground (like your truck), etc. But OTOH the antenna is going to really perform roughly like they say and it's more a matter of how much you give up using junk coax and Rat Shack PL259 connectors. If there's 'free' energy to be had it's fixing up installations if ask me, a lot of people are giving away RF energy just heating the breaking down dielectric in their old coax with half a dozen sharp bends than they could ever get with a higher gain antenna.

Thanks Dave,
While I'm no Cebik, I do understand what you were explaining here in your posts. I've played with modeling software (EZNEC) in the past while home-brewing HF antennas. My point was essentially what you'd reiterated in your last paragraph. Rarely are mobile antennas mounted in optimal conditions, or the conditions in which they're tested. If they are physically tested, they certainly don't get tested on all vehicle makes and models or at every conceivable mounting option on a given vehicle. And, it could be argued as to whether or not it's worth the trouble/fuss in finding an antennas absolute best application to any given vehicle for the minimal dB difference that would be realized.....especially with commercially built mobile FM antennas. They are about as "plug and play" as any antennas you'll find in this hobby.

 

[RSB]

bringing this thread back...

I understand the advantages and disadvantages to using a high gain antenna when transmitting, however, I don't understand logically how a higher gain antenna is better when receiving. For example:

a 0dB gain antenna radiates in a sphere-like pattern. When 'squished', the gain is increased and therefore, outgoing transmission range is increased. However, as I understand it, at the same time, incoming reception is also increased.

how is this possible? The way I see it, when the sphere is squished, the range is more limited. As you can see in the drawing below, the signals need to travel further than before (RED area). The reception is actually decreased, from what I can see. The only exception to this is at the bulge (BLUE arrow), where the reception would be increased. But this is a small part of the sphere overall, as you can see...

I suspect I'm just over-thinking this. Anyone willing to help clear things up for me would be appreciated. Thank-you!