3dB gain vs. 6dB gain antenna

[Greg43545]

I just received my MXT400VP3 Micromobile Bundle with a MXTA25 3dB Gain Antenna and plan on using it as a base unit, mounting the antenna approximately 25 feet up on a Tri-pole TV antenna tower.

 

I know there is an MXTA11, and an MXTA26 6dB Gain Antenna available for use with the MXT400.

 

Is there that much difference Performance wise, that it would be advantageous for me to upgrade my antenna from the 3dB gain to the 6dB gain antenna?   By performance I'm speaking of increased range and clarity of signal.

 

If so, which would be the best to choose? The MXTA11, or the MXTA26 ?

 

 

[dwmitchell61]

Just bought the same package myself!

 

The gain will flatten the signal radiated, less radiated vertically. more horizontally as the gain goes up. this raises the ERP (Effective Radiated Power). For every 3dB in gain, the ERP is doubled.

 

For our radios, channels 15-22 and RP15-RP22 the limit is 50W transmitter power.

 

BUT... For channels 1-7 the FCC limits the power to 5W ERP (Effective Radiated Power).  So if we run at 5W and have an antenna gain of 9dBi, the ERP will be 30W! 

 

Check this out..... https://www.calculators.tech/erp-calculator

 

I guess the point is, be cautious about high gain antennas on a BASE station, as you may "stomp" all over other users and cause them to raise a complaint. The FCC says use the minimum power needed to communicate for this reason.

 

Everyone seems to recommend LMR400 cable to reduce cable losses. It is expensive, but if you have a tower it may be worth it.

[berkinet]

This has been addressed before. But, in very basic terms UHF range is limited to line of sight. If a basic 1/4 wave antenna (I.e. no gain) covers the line of sight area, adding more gain will make no difference. On the other hand, if you have a clear view to the horizon, and will be contacting stations near the edges of that coverage, extra gain might help.  Keep in mind, this is not CB and the radio propagation is quite different.

[Greg43545]

Just bought the same package myself!

 

The gain will flatten the signal radiated, less radiated vertically. more horizontally as the gain goes up. this raises the ERP (Effective Radiated Power). For every 3dB in gain, the ERP is doubled.

 

I guess the point is, be cautious about high gain antennas on a BASE station, as you may "stomp" all over other users and cause them to raise a complaint. The FCC says use the minimum power needed to communicate for this reason.

 

Everyone seems to recommend LMR400 cable to reduce cable losses. It is expensive, but if you have a tower it may be worth it.

 

Thanks for the reply and info, I very much appreciate it.

 

Fortunately we live in a rural area and, to my knowledge. there are not many GMRS radios that a high gain antenna would " stomp " on to raise a complaint. I've been studying the FCC requirements as far as power usages and have made a color coded chart for my personal use so I will know what power to set the radio on for what channels until it becomes second nature to me.

 

As for the LMR400 cable. I have read that Rg58u coax cable is the cable of choice for GMRS, that LMR400 doesn't flex easily and is quite thick, about 1/2 inch. Since I'm going to have to run the coax down the tower into the house attic, and down into the room I was planning on setting up the radio, I was going to use the Rg58u instead.  Thoughts anyone?

 

Also a few questions if I may?

 

1) Have you set your MXT400 up yet?

 

2) Are you using the 3dB or 6dB Gain Antenna?

 

3) How do you like it ( Both radio and/or antenna ), and is it performing to your expectations?

[Greg43545]

If a basic 1/4 wave antenna (I.e. no gain) covers the line of sight area, adding more gain will make no difference. On the other hand, if you have a clear view to the horizon, and will be contacting stations near the edges of that coverage, extra gain might help.

 

Living in a rural area, most surrounding terrain is farmland fields with some woods interspersed for miles around, line of sight will be much better than say what the urban or suburban terrain would be.

 

I would like the capability of being able to contact my handheld mobile(s) locally, AND being able to use a repeater 13 miles away to contact other base and mobile radios " near the edges of that coverage " IF necessary.

 

With that being said, in your opinion, would the 6dB gain antenna be advantageous to what I'm proposing the radio to perform?

[berkinet]

...I guess the point is, be cautious about high gain antennas on a BASE station, as you may "stomp" all over other users and cause them to raise a complaint. The FCC says use the minimum power needed to communicate for this reason....

 

You, and the FCC are both correct in principle. However, in practice a 3 or even 6db difference with FM transmission is not going to overpower someone like it would on CB.  There are a few things that affect that. First is capture effect. With FM transmission (remember GMRS is FM,  CB is AM) you tend to hear the most powerful only and nothing else on the same frequency - it is called capture effect. However, since distance is a huge factor is signal strength, proximity usually counts far more than a 3db or 6db difference in power.  Second, and more important, on UHF communications the common rule is make sure the channel is clear before you transmit. This can be an issue when there is more than one repeater on a frequency, using different PL/DCS tones. You won't hear if the channel is busy. So, in practice people will tell you you were cut off, or you just try again a minute later.  Some higher end radios will have a channel busy lamp that lets you see if the channel is busy even if you are not hearing something.  And, last, again unlike CB, because UHF is line of sight, the number of stations you can interfere with is limited.

 

It sound confusing, but you will get used to it and in the end it is not a big problem.  I would say go with the lowest gain antenna that suits your needs.

[berkinet]

...in your opinion, would the 6dB gain antenna be advantageous to what I'm proposing the radio to perform?

I would say (and have said to anyone who will listen) start with a cheap 1/4 wave antenna (I.e. no gain).  Then, if that doesn't work, take a look at something else. 9 time out of 10 you will just stick with the 1/4 wave.

[BoxCar]

As to which coax, look them both up and note the loss figures. Calculate the length needed to go from you antenna to your radio and then apply the loss figures. Simple math, but you want the cable with the least loss as it means more power to the antenna. For the most part, you can ignore the connectors and their loss.

[Greg43545]

I would say (and have said to anyone who will listen) start with a cheap 1/4 wave antenna (I.e. no gain).  Then, if that doesn't work, take a look at something else. 9 time out of 10 you will just stick with the 1/4 wave.

 OK thanks. The Bundle came with the 3dB antenna, so I'll set that up and see if it meets our needs. If not, I'll step up to the 6dB antenna.  Thank you.

[Lscott]

 

BUT... For channels 1-7 the FCC limits the power to 5W ERP (Effective Radiated Power).  So if we run at 5W and have an antenna gain of 9dBi, the ERP will be 30W! 

Ah no. When you see gain spec of 9dbi that’s a gain over an isotropic antenna that doesn’t exist. It’s for reference only. What you really want is a gain spec over a dipole,  or a simple quarter wave, which is the smallest antenna that can be physically built. The difference in gain between the two is 2.15db. So the real practical gain is now 6.15db.

 

But wait, we’er not done yet! Now you have to figure in your coax loss. For a typical run of 75 feet of LMR400 at 450MHz, close enough to the GMRS band at 462 to 467, the loss is another 2.03db. So your real gain is around 4.82db, which is a factor of 3.03 increase. For 5 watts in it looks like you have an effective radiated power of 15.17 watts.

[PRadio]

 

But wait, we’er not done yet! Now you have to figure in your coax loss. For a typical run of 75 feet of LMR400 at 450MHz, close enough to the GMRS band at 462 to 467, the loss is another 2.03db. So your real gain is around 4.82db, which is a factor of 3.03 increase. For 5 watts in it looks like you have an effective radiated power of 15.17 watts.

 

It's worse than that, he said he will be using Rg58u. 

[Jones]

RG58u... just no.  That's OK for less than 15 feet in a mobile environment, but not for a base station.  If you are going to be running down a tower, and into the house, and into the next room, you will have 60-100 feet before you know it, and that thin RG58u will have no more signal left at the end of the cable.  It is WAY too lossy for long cable runs.

 

Use LMR-400 at a minimum if you are planning a run of over 50 feet of cable.

[Lscott]

It's worse than that, he said he will be using Rg58u. 

I’ll leave that as an exercise for the student to figure out the system gain as they instructor used to say.

[Lscott]

So the real practical gain is now 6.15db.

 

A small typo I made. The gain should have been 6.85db. The rest of the numbers should be correct.

[dwmitchell61]

Thanks for the reply and info, I very much appreciate it.

 

Fortunately we live in a rural area and, to my knowledge. there are not many GMRS radios that a high gain antenna would " stomp " on to raise a complaint. I've been studying the FCC requirements as far as power usages and have made a color coded chart for my personal use so I will know what power to set the radio on for what channels until it becomes second nature to me.

 

As for the LMR400 cable. I have read that Rg58u coax cable is the cable of choice for GMRS, that LMR400 doesn't flex easily and is quite thick, about 1/2 inch. Since I'm going to have to run the coax down the tower into the house attic, and down into the room I was planning on setting up the radio, I was going to use the Rg58u instead.  Thoughts anyone?

 

Also a few questions if I may?

 

1) Have you set your MXT400 up yet?

 

2) Are you using the 3dB or 6dB Gain Antenna?

 

3) How do you like it ( Both radio and/or antenna ), and is it performing to your expectations?

No. I just received it and a power supply to run it at my home. I have been researching the operating rules and antenna options before setting it up. There is also RG-58X low loss available, also.

 

I bought a ground plane kit to use with the 3dB antenna it came with and hope to get a Deep Recon G1 kit for "bug out", etc.i will mount the antenna on a camera tripod for that operation.

 

I was trying to find specs for the transmit power on lo and mid settings.

[Greg43545]

No. I just received it and a power supply to run it at my home. I have been researching the operating rules and antenna options before setting it up. There is also RG-58X low loss available, also.

 

I bought a ground plane kit to use with the 3dB antenna it came with and hope to get a Deep Recon G1 kit for "bug out", etc.i will mount the antenna on a camera tripod for that operation.

 

I was trying to find specs for the transmit power on lo and mid settings.

 

First of all I would like to thank you for your professional response, some here could take a lesson from you on " civility ".

 

I assume you're going to go with the LMR400 cable, how many feet do you estimate it will take to set up your antenna to your radio, and what are you estimating the cost will be for the coax?

 

What make and model ground plane kit are you considering and where did you find it? I would like to research it as well.

[Lscott]

I just received my MXT400VP3 Micromobile Bundle with a MXTA25 3dB Gain Antenna and plan on using it as a base unit, mounting the antenna approximately 25 feet up on a Tri-pole TV antenna tower.

 

I know there is an MXTA11, and an MXTA26 6dB Gain Antenna available for use with the MXT400.

 

Is there that much difference Performance wise, that it would be advantageous for me to upgrade my antenna from the 3dB gain to the 6dB gain antenna?   By performance I'm speaking of increased range and clarity of signal.

 

If so, which would be the best to choose? The MXTA11, or the MXTA26 ?

You should consider using a different antenna. The MXTA25 is listed as a 5/8 wave design.

 

https://midlandusa.com/product/micromobile-mxta25-3db-gain-ghost-antenna/

 

From modeling this antenna has a net gain LOSS over a simple 1/4 wave, or 1/2 wave dipole antenna. Where you want the signal strength at the maximum is along the ground and not shooting up in the air at an angle.

 

https://www.w8ji.com/VHF%20mobile%20vertical.htm

 

The low loss cable you mentioned I found a datasheet for it from Belden Cable. The loss spec is 4.9db per 100 feet compared to LMR-400, 2.7db per 100 feet, at the same frequency of 450MHz which is very close to the GMRS frequencies.

 

https://catalog.belden.com/techdata/EN/7808WB_techdata.pdf

 

https://www.timesmicrowave.com/DataSheets/CableProducts/LMR-400.pdf

 

For the MXT400 radio there is an FCC test report you might find of interest. On page 7 it has the high and low power measurements.

 

https://fccid.io/MMAMXT400/Test-Report/Test-Report-3120260.html

[Lscott]

I forgot to include a datasheet for regular RG-58U cable.

 

https://catalog.belden.com/techdata/EN/9201_techdata.pdf

 

As you can see the loss is almost double at  8.4db at 400MHz. The losses go up as the frequency increases. One table I have shows it at 10.6db per 100 feet at 450MHz. With this kind of loss one would do better sending smoke signals.

[berkinet]

...As you can see the loss is almost double at  8.4db at 400MHz. The losses go up as the frequency increases. One table I have shows it at 10.6db per 100 feet at 450MHz. With this kind of loss one would do better sending smoke signals.

No disagreement. Only a reminder to readers of a point @Jones made earlier. There is a place for thin cables, particularly in a mobile environment. Also, in a case where the radio is at a window and the antenna line goes out the window and a few feet up to an antenna. 

 

Noted in these forums before, but always important to keep in mind: Perfect is the enemy of good.  If someone deployed 50 feet of RG58 to their antenna and can reach all the repeaters and mobiles they want, then that is probably a good time to stop.

[marcspaz]

...some here could take a lesson from you on " civility ".

 

Did I miss something? Seems like civil responses here.

[Greg43545]

Did I miss something? Seems like civil responses here.

 

There were several replies made that, instead of being instructional, were snarky cynicism and I removed them from this thread and blocked them as well, that is why you don't, or didn't, see them.

 

I'm here to ask questions and learn. I can take "constructive" criticism if that is an intention of the poster, but don't feel it's necessary to be berated.

 

You and berkinet have always responded with positive instructions and I appreciate it - thank you. 

[Lscott]

No disagreement. Only a reminder to readers of a point @Jones made earlier. There is a place for thin cables, particularly in a mobile environment. Also, in a case where the radio is at a window and the antenna line goes out the window and a few feet up to an antenna. 

 

Noted in these forums before, but always important to keep in mind: Perfect is the enemy of good.  If someone deployed 50 feet of RG58 to their antenna and can reach all the repeaters and mobiles they want, then that is probably a good time to stop.

One point not mentioned much are the gain figures work both ways. That is on transmission and reception. It pays to optimize the losses. A 5 watt radio likely is about as sensitive as a 40 to 50 watt one. You can run into cases where spending more money on a higher power radio to make up for the power loss but it does nothing to increase the receive signal strength. If you can’t hear the other station it doesn’t matter how much power you run. This could be the difference between using a 5 watt handheld with a roof mount antenna, or spending a lot more money on a high power mobile radio.

 

I think enough information is here where choices can be made that fit budget and physical installation requirements. There shouldn’t be any really big surprises how the final system will perform.

[berkinet]

There were several replies made that, instead of being instructional, were snarky cynicism and I removed them from this thread and blocked them as well, that is why you don't, or didn't, see them.

 

I'm here to ask questions and learn. I can take "constructive" criticism if that is an intention of the poster, but don't feel it's necessary to be berated....

 

I did not see the posts in question. However, I am very surprised to hear about snarky criticism on this forum. This is probably the best behaved online group or forum I have ever participated in. Yes,  I have seen a couple of disagreements, but even then, the discussion was always fact based and never mean spirited.  Is it possible you are being a little sensitive?  That has actually happened before, and when the issue was noted and people explained their views and why they had posted them, the problems vanished.  And, on a few rare occasions, the criticism of the poster was justified, behavior changed and we all moved on.

 

Just my 2¢ worth.

[Greg43545]

 Is it possible you are being a little sensitive?  

 

 No.

[Jones]

There were several replies made that, instead of being instructional, were snarky cynicism and I removed them from this thread and blocked them as well, that is why you don't, or didn't, see them.

Now Mr. E., I am an advanced member that has been here for almost 4 years, and I am not able to 'block people' or 'remove posts' on this forum.  Perhaps you are thinking of some other forum.

 

...or perhaps you are full of hot air, and/or being overly sensitive.  That's as snarky as it is going to get around here.