Tuning Antenna?

[sirdonbWRYY778]

After searching this forum and Google, I couldn't find what I was looking for so I decided to bring the question here to the hive mind.

Can you tune a stealth antenna like the MXTA25?

I bought this antenna and mount package from Amazon. I'm using the Surecom SW-102s power/SWR Meter to check my transmission SWRs and power output. The reason I want to know if the stealth antenna can be tuned is because I'm getting the following SWR readings:

155.0000 - 5.94 on high power 7.01 on low power
240.0000 - 6.07 on high power 6.25 on low power
370.0000 - 3.53 on high power 3.61 on low power
440.0000 - 1.00 on high power 1.00 on low power

If it matters, these readings were taken in my driveway where the antenna has at least a 20ft sphere from obstructions. In addition, the coax cable is 5m/16.5ft. I am planning on shortening the coax to eliminate the extra length I don't need in hopes that it will help the SWR readings. But I still need to know if I can tune this antenna.

Picture of the antenna and mount on my vehicle. Location is on the driver side hood as close to the windshield as possible.

I will end this post with the following statement:
While I'm not completely new to radios, I spent 6 years in the military operating various radio systems and I've always had a CB radio in my personal vehicles until now, I am new to this level of radio equipment on the civilian side. I currently only have my GMRS license and do plan on getting my HAM license in the near future. So I will be spending most of my time transmitting on the FRS/GMRS channels but I would like the peace of mind knowing that if I had to transmit on Ham frequency in an emergency.

 

[KevinC]

Seeing how that antenna is a single band (UHF) and specifically tuned for GMRS the answer to your question is no.

 

[mmckenna]

155.0000 - 5.94 on high power 7.01 on low power
240.0000 - 6.07 on high power 6.25 on low power
370.0000 - 3.53 on high power 3.61 on low power
440.0000 - 1.00 on high power 1.00 on low power

That looks exactly like what I'd expect for that antenna. As KevinC pointed out, that's a UHF antenna and it's going to work on the UHF band.

but I would like the peace of mind knowing that if I had to transmit on Ham frequency in an emergency.

That antenna should work well enough if you stick to the 70cm and GMRS bands.

If you want to transmit on the 2 meter amateur radio band, then you'll need a suitable antenna.
This antenna will work well on GMRS, pretty well on the amateur 70cm band, and pretty well on the 2 meter band. I'm using this antenna on my work truck connected to a multiband mobile and have used it on 2m/70cm as well as GMRS:

Pulse Larsen NMO150/450/758 Antenna

The Pulse – Larsen NMO150/450/758 is a Tri-band Whip antenna with a spring base. *This Antenna Requires a NMO Mount. Please click here to find all Thru-Hole NMO mounts or Magnetic NMO Mounts we carry.

www.arcantenna.com

 

[sirdonbWRYY778]

Interesting as it is marked/labeled on the bottom band as being dual band for 136-174 and 400-480.

I understand this might be because it's a cheap clone and not a more expensive name brand.

However are there any stealth antennas that are dual band and tunable? I will be getting a whip style antenna in the future as well once my budget opens up more so I can make the investment.

 

[KevinC]

Interesting as it is marked/labeled on the bottom band as being dual band for 136-174 and 400-480.

I understand this might be because it's a cheap clone and not a more expensive name brand.

However are there any stealth antennas that are dual band and tunable? I will be getting a whip style antenna in the future as well once my budget opens up more so I can make the investment.

Not sure where the dual band info came from.

FEATURES​

Specific Frequency: 462 MHz
Gain: 3dB
5/8 wave antenna
3.5 inches in length
1.5 inch base diameter
Mobile Antenna Mounting Type: NMO
Maximum Power: 120 W
Color: Black
Antenna mount NOT included
Compatible with MXT105, MXT115, MXT275, MXT400, MXT500, MXT575

MicroMobile® MXTA25 3dB Gain Ghost Antenna

Ensure a solid connection to your vehicle with the Midland ghost antenna. The MXTA25 3dB gain antenna is 3.5 inches in length and 1.5-inch base diameter.

midlandusa.com

 

[sirdonbWRYY778]

Dual band came from the product listing on Amazon.... https://a.co/d/6PzUjT2

Description
Specifications:
Frequency Range: VHF 136-174MHz, UHF 400-470MHz
Feature: Lip Fixed Bracket Mounting
Compatible with: FRS GMRS MURS Radio; Amateur Radio; Vehicle Mobile Radio; Ham Radio; Two Way Radio; Public Radio Scanner; Marine VHF Radio;
Compatible with Amateur Mobile Radio, Ham Radio, FRS GMRS Two Way Radio: BTECH, ICOM, Kenwood, Midland, Yaesu, TYT, AnyTone, Radioddity, Cobra, Uniden, President, Galaxy Audio, Pro Trucker, Stryker;
VSWR: <2.0;
Impedance: 50 ohm;
Direction: Omni-directional;
Antenna Interface Connector: UHF PL259 Male 50 Ohm Connector
Antenna Mounting Connector: NMO Mount Connector
Cable Connector: UHF PL259 Male 50 Ohm Connector
Cable: RG58 50 Ohm Coaxial Cable
Length: 5m / 16.5 Feet
Impendence: 50 Ohm

 

[mmckenna]

Yeah, you are talking about one antenna, and showing a photo of something else. The Midland antenna is not dual band.

If that is one of those Chinese dual band salt shaker antennas, I'm still not surprised. For VHF, a 1/4 wave antenna (smallest efficient design) is going to be 18 inches or so. Cramming 18 inches of wire into a 3" tall package isn't going to work. UHF is looking for around 6", and that's usually achievable.

No matter what the seller is telling you about the antenna you showed in your photo, it is not going to work well on VHF. The laws of physics take priority over what the sticker says.

 

[sirdonbWRYY778]

I used the MXTA25 as an example in my original question as I figured most people would be familiar enough to know the STYLE antenna I was asking about.
The pictured antenna is the actual antenna I have currently. In addition I provided the link to the antennas listing in my original post.

 

[mmckenna]

I used the MXTA25 as an example in my original question as I figured most people would be familiar enough to know the STYLE antenna I was asking about.

Got it. The 'style' is one thing. The internal design under the cover is different.

The pictured antenna is the actual antenna I have currently. In addition I provided the link to the antennas listing in my original post.

The link didn't work for me, so all we had to go off of was the part number.


Like I mentioned above, cramming 18" of antenna into a 3 inch tall can doesn't come close to reality. The only manufacturers of these dual band low profile antennas are out of China and no legit commercial antenna manufacturer sells such a thing. The reason they don't is because they do not work. Chinese companies can stick whatever sticker they want on the antenna, but the sticker isn't going to change the laws of physics. It's meant solely to get people to buy them. No matter what you do, that will never ben a well performing antenna on VHF.

If you want the low profile for GMRS, and that antenna works well for you, then stick with it. Keep a 1/4 wave VHF antenna under the seat for those times you want to work VHF.

Or, get the antenna I linked to (or something similar) and use that.

3" tall antenna isn't going to work well on VHF, and the high SWR isn't a surprise to any of us.

 

[mmckenna]

One additional item I'd add:

Since we cannot see the full picture, mount, vehicle, there are absolutely other things that will impact tuning. It's likely not enough to cause what you are seeing, but:
Those sorts of mounts do not provide a good ground plane under the antenna. A proper ground plane is important to the proper function of the antenna, and will directly impact the SWR you see.

Without seeing the whole picture, there may be other variables in your installation that are impacting its performance. Proximity to any other body steel/aluminum, deep tinted windows, other antennas, even AM/FM, proximity to electrically defrosted windows, how well the mount is attached, etc. Those all can potentially create issues.

 

[sirdonbWRYY778]

One additional item I'd add:

Since we cannot see the full picture, mount, vehicle, there are absolutely other things that will impact tuning. It's likely not enough to cause what you are seeing, but:
Those sorts of mounts do not provide a good ground plane under the antenna. A proper ground plane is important to the proper function of the antenna, and will directly impact the SWR you see.

Without seeing the whole picture, there may be other variables in your installation that are impacting its performance. Proximity to any other body steel/aluminum, deep tinted windows, other antennas, even AM/FM, proximity to electrically defrosted windows, how well the mount is attached, etc. Those all can potentially create issues.

Understood. As stated, I am planning on getting a whip style antenna, the one you linked or similar. I also understand that I won't get the same range with the 3in antenna vs something taller and my chosen placement may affect range as well.

As I'm posting from my phone, the site wouldn't let me upload a couple of the pictures I wanted to because they were too large for the server. The vehicle is a 1998 Dodge Durango.

The antenna mount slides over the lip of a panel and is held in place by 4 set screws. On the under side where the antenna is attached, it came with a thin silver metal plate that fits in that space snugly (I'm assuming this plate helps create a ground plane for the antenna). The entire bracket looks like it's powder coated, no bare metal showing other than the inside of the screw holes. It also came wit a similar rectangular strip of metal that I placed between the set screws and metal of the hood.

Factory radio antenna was removed in favor of a hidden antenna in the A pillar on the passenger side. It's mounted on the hood of the Durango on the driver side as close to the windshield as I could get to not impende my view.

Coax is routed behind the door hinges in a void in the body to the interior coming out behind the kick panel before it goes up into the dash to connect with the radio that tucked into the space that used to be the ashtray.

As I'm going to be shortening the coax this weekend, I'll make sure that before I do, I have it routed to avoid as much electrical interference as possible.

 

[sirdonbWRYY778]

Thank you to everyone who posted to this point for helping me figure out if this antenna was tunable or not.

 

[prcguy]

Here is something to consider when trying to tune an antenna for a new frequency. Some antennas, mostly whip type can be adjusted a little by pulling the whip out of the mount to lower the frequency or cutting it to raise the frequency. Some short phantom types have a tuning screw on the side but those have a very limited tuning range.

The frequency ranges are usually mapped out by the mfr and poured over by over zealous marketing folks so they can advertise the most frequency range over the competition. It’s highly unlikely you will ever get an antenna to work very well if at all beyond its advertised frequency range. If it could the marketing dept would have been on that already.

I noticed the SWR readings taken on the MXTA25 and there is something wrong with the result as SWR is independent of power, if an antenna has a 3.2:1 match that’s what it is at any power level unless the power is so high the antenna is heating up and changing. I would not trust the SWR readings above except to the point that the antenna will not work at 240 or 370MHz. I’m also a little curious why you want the antenna to work there. Experimenting with UHF satellites perhaps?

 

[mmckenna]

As I'm going to be shortening the coax this weekend, I'll make sure that before I do, I have it routed to avoid as much electrical interference as possible.

Before doing that, read below:

The antenna mount slides over the lip of a panel and is held in place by 4 set screws. On the under side where the antenna is attached, it came with a thin silver metal plate that fits in that space snugly (I'm assuming this plate helps create a ground plane for the antenna). The entire bracket looks like it's powder coated, no bare metal showing other than the inside of the screw holes. It also came wit a similar rectangular strip of metal that I placed between the set screws and metal of the hood.

The plate is likely included to spread out the clamping force of the mount to prevent damage to the body panel.

That's great if you want to prevent damage to the body panel. Its awful if you want a proper ground connection. You'd want the set screws to actually bite into the metal of the body panel, through the paint. But that will damage the panel and it often eventually corrodes/rusts and you'll have to redo it.
My other concern would be the base of the antenna hovering a few inches above the body panel. That's not helping things either. There's likely no good ground connection on that mount. Those things are usually intended for mounting ground independent antennas for ham radio operators that don't want to do a full/proper install.

Two things you want to consider:
1. Any mobile antenna is going to perform better with at least a quarter wavelength of ground plane under the antenna in all directions.

Mounting the antenna offset to one side is going to result in not only an insufficient ground plane, but also a lopsided one. That will impact tuning and will impact radiation pattern. Even with a proper whip antenna, tuning for low SWR may be difficult in that location. The lopsided ground plane is also going to make the antenna directional away from the vehicle body. This is one of the reasons you will see most public safety vehicles with the antenna mounted in the center of the roof, or on sedans sometimes on the center of the trunk lid.

2. Placing the antenna close to the A pillar is going to impact tuning and impact radiation pattern. The metal A pillar can reflect the RF and cause issues.
Moving it away from the A pillar may improve performance.

Ideally, you want the antenna mounted on the roof of the vehicle, in the center, or at least ~19 inches in from any roof edge. That'll give you a suitable ground plane, an even radiation pattern, and will make tuning the antenna much easier. A permanent mount is your best choice. It does require drilling a hole in the roof. Less desirable option is a magnetic mount in the center of the roof.

Those compromise clamp on mounts are never going to work well and ideally would only be used with a ground independent antenna (but it's still going to suffer). At some point you need to decide if you want performance from your antenna system, or are you more concerned with ease of installation and looks. Your truck, your antenna, your radio, your choice. I've always gone for performance, and never regretted doing the permanent install on the roof. But that's just me….

 

[sirdonbWRYY778]

I would not trust the SWR readings above except to the point that the antenna will not work at 240 or 370MHz. I’m also a little curious why you want the antenna to work there. Experimenting with UHF satellites perhaps?

Not experimenting with anything at the moment, however my radio is rated to work in those bands. So I was checking the antenna through all 4 bands my radio is capable of.

I'm starting to see and understand that my antenna is likely a case of false advertising by the manufacturer and is really only for the 400-480mhz range. As is backed up by my SWR readings.

Ifeally, you want the antenna mounted on the roof of the vehicle, in the center, or at least ~19 inches in from any roof edge. That'll give you a suitable ground plane, an even radiation pattern, and will make tuning the antenna much easier. A permanent mount is your best choice. It does require drilling a hole in the roof. Less desirable option is a magnetic mount in the center of the roof.

With the full length custom roof rack I'll be making to go on top of the Durango, I'll see how I can work the antenna mount into the design. However it'll still be out on the edge of the roof.

Those compromise clamp on mounts are never going to work well and ideally would only be used with a ground independent antenna (but it's still going to suffer). At some point you need to decide if you want performance from your antenna system, or are you more concerned with ease of installation and looks. Your truck, your antenna, your radio, your choice. I've always gone for performance, and never regretted doing the permanent install on the roof. But that's just me….

As I'll mainly be operating in the GMRS frequencies and the SWR readings in that range are already showing as 1.0:1, I think I'll be fine for the time being. As I design my roof rack, I'll take the advice given into account as I have to balance ascetics, functionality, maintenance, and performance between the antenna and roof rack.

The main end goal for the Durango is to be a combo overlanding and bug out daily driver for myself and my family. So some compromising has to be done to fit everything together seamlessly while making it look like it rolled off the showroom floor like that.

 

[mmckenna]

The main end goal for the Durango is to be a combo overlanding and bug out daily driver for myself and my family. So some compromising has to be done to fit everything together seamlessly while making it look like it rolled off the showroom floor like that.

Yep, absolutely a challenge.

If you are going to be fabricating your own roof rack, adding a tab with a 3/4" hole will make a handy place to install an NMO mount.

 

[sirdonbWRYY778]

Yep, absolutely a challenge.

If you are going to be fabricating your own roof rack, adding a tab with a 3/4" hole will make a handy place to install an NMO mount.

Yup, so I'll be adding it to the overall design. I'm going for that low profile style you see on Toyotas. Now I just have to figure out exactly where to mount it on the rack.

 

[mmckenna]

Now I just have to figure out exactly where to mount it on the rack.

In the center (both left/right as well as front/back) would give you the ground plane you need. But that may not work with the design.

Doesn't have to be perfect for it to work passably. I've seen people mount them on the front or rear edge of the rack, but centered on the left/right plane and it's worked OK.

Other challenge is getting the coax into the vehicle without it getting damaged/pinched in doors/windows.

 

[sirdonbWRYY778]

In the center (both left/right as well as front/back) would give you the ground plane you need. But that may not work with the design.

Doesn't have to be perfect for it to work passably. I've seen people mount them on the front or rear edge of the rack, but centered on the left/right plane and it's worked OK.

Other challenge is getting the coax into the vehicle without it getting damaged/pinched in doors/windows.

Yeah that location definitely won't work on the design. I may try to center it on the front above the windshield depending on exact how I do the design and what material I use for that side of the rack. And to assist the problem, I'll look for the best no ground plane antenna I can afford when the time comes.

Getting the coax into the cab without damage will be the easy part of the whole project. I've done it before with other vehicles I've owned.