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Mobile Antenna Placement and Tuning


jsurles
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Hello,

 

I'm running the following equipment:

MXT275
MXTA26 on a tunk lip mount.

I drive a small car (Honda Fit) and am not too keen on making any holes.  I have had magnetic bases in the past which have worked wonderfully.. but unless you're more attentive than I am.. after time you start getting "wear" marks on your roof.. or worse, rust if your not careful.
 

Because of that, I thought I'd try the trunk lip mount.  It seems to work fine.. but I'm curious if there are any negative consequences to how it's angled.  The only place I can seem to fit it on my hatch is towards the side, so it angles to one side of the vehicle or the other.  I can't put it in the middle because it will hit the roof when I open the hatch.  I could also put it in the front on the hood, but it would do the same thing (but he lower).
 

Does anyone have any input on antenna placement like this?  What are some other options (including a magnetic mount) that might be better.. and potentially how much better?  For instance if a mag mount in the center of my roof would work 10x better I might try it.. but if it may only marginally approve, I might not.

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Depending on amount of metal, shapes etc. You get distortions in the radiation pattern.

 

Ground plane is important because it is the other half of an antenna. 1/2 and larger are marketed sometimes as no groundplane or non groundplane dependent. It is because at the 1/2 wave point the sinoid is partially complete and not reflected back into the ground plane.

 

As well as directionality of the antenna. You are shaping the radiation pattern based off the ground plane and its contiuity. Basically, you can get better recieve or nulls in the radiation pattern.

 

Take it all with a grain of salt. Because you may or may not perceive the differences.

 

In all any antenna is better then no antenna. The mobile install is already a compromised install.

 

post-2261-16181713360756_thumb.jpg

 

Sent from my SM-G975U using Tapatalk

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Thanks guys.. just trying to understand my options. As far as the ground plane, how is my radiation affected by a non perpendicular angle on the curved portion of the roof. Would it be better to use an adjustable mount so that the mast is vertical?

Good Day.

 

Yes, the angle of the antenna relative to the ground plane will affect the antennas radiation pattern and its impedance. As far uniform horizontal coverage is concerned, the more vertical the better. However, when using a low gain antenna the angle is not as critical as it would be with a high gain antenna. Low-gain antenna radiation patterns are closer to that of a fat-donut. If your SWR is within an acceptable range I would put your radio and antenna to the test. If it does what you need it to do, leave it be. If not you will need to experiment a bit until you find the placement, orientation and mounting that works for you.

 

 

Michael

WRHS965

KE8PLM

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I think the antenna itself is your limiting factor.

It's basically a "rubber duck" for a car.

I've read posts about these antennas being close to worthless.

 

If this antenna works OK for your day-to-day use then just keep it as is.

You could have a good mag mount for the times when you need more range.

Just stash it in the back and put it on when you need it. 

 

Vince

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Good Day.

 

Yes, the angle of the antenna relative to the ground plane will affect the antennas radiation pattern and its impedance. As far uniform horizontal coverage is concerned, the more vertical the better. However, when using a low gain antenna the angle is not as critical as it would be with a high gain antenna. Low-gain antenna radiation patterns are closer to that of a fat-donut. If your SWR is within an acceptable range I would put your radio and antenna to the test. If it does what you need it to do, leave it be. If not you will need to experiment a bit until you find the placement, orientation and mounting that works for you.

 

 

Michael

WRHS965

KE8PLM

This antenna is advertised as a 6db gain antenna.. would that be considered low or high gain?  I assume it's high-gain..   As far as an acceptable SWR range, what is considered acceptable?  It seems like some places say under 1.5:1, others under 2:1, while others seem to say nothing above 1.1:1 is acceptable.  Currently using a 15W radio, but if I go to a 40W or 50W, would this SWR range become more "dangerous"?

 

Currently I can easily "hear" my local repeater that is nearly 28miles away.. and I can key it up, but they can't hear me worth a a hooey.. with getting a near perfect SWR on the repeater channels.. I may just need more juice.

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If this antenna works OK for your day-to-day use then just keep it as is.

You could have a good mag mount for the times when you need more range.

Just stash it in the back and put it on when you need it. 

What do you consider a "good" mag mount?

 

My biggest concern is doing damage to the radio with the SWR being between 1.5-1.8.. if that's acceptable.. I'm okay with the antenna as is.

 

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Just remeber perfect SWR doesn't mean you have a resonant antenna.

 

Sent from my SM-G975U using Tapatalk

True True.. I'll order up an NMO magmount and put it on the roof center-ish.. and see how much of a difference it makes.  That's an inexpensive test.

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This antenna is advertised as a 6db gain antenna.. would that be considered low or high gain? I assume it's high-gain.. As far as an acceptable SWR range, what is considered acceptable? It seems like some places say under 1.5:1, others under 2:1, while others seem to say nothing above 1.1:1 is acceptable. Currently using a 15W radio, but if I go to a 40W or 50W, would this SWR range become more "dangerous"?

 

Currently I can easily "hear" my local repeater that is nearly 28miles away.. and I can key it up, but they can't hear me worth a a hooey.. with getting a near perfect SWR on the repeater channels.. I may just need more juice.

Yes I would consider a 6dBi antenna a high gain antenna as far as mobile antennas are concerned. Some base antennas though may reach 9, 12 and above.

 

What is acceptable in SWR varies in what equipment you are using. Some pre-tuned antenna companies advertise their antennas to only be <1.7:1, while I have read on this forum someone assert that one of the GMRS radio manufacturers won’t warrant their products if connected to an antenna system greater than 1.5:1. As a radio owner you want to get it as low as you practically can, while also realizing that minute changes mean little in the grand scheme of things.

 

An SWR of anything other than 1:1 means power is being reflected back from the antenna to your radio. A small amount is OK. A large amount of reflected power could damage it. A high SWR also means you are unnecessarily wasting power in your coax as it bounces back and forth from one end of your cable to another.

 

No, a 2:1 SWR is not terrible, but it still translates into 11% of the power sent to the antenna being reflected back to your radio. Whereas a 1.5:1 translates only to 4% and a 3:1 translates to 25% power reflection. Here is a document worth reading: http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf.

 

If you have a radio capable of operating at high power but your SWR is not optimal, yes it is wise to operate the radio at lower power. Doing so ensures that the outputs of your radio are not subjected to voltages and more heat than they are rated for.

 

Personally I strive for <1.5:1. I consider myself blessed if I get less than that, and I back the power down if it is above that.

 

Oh, and by the way, a low SWR does not mean your antenna is radiating optimally, it only means you antenna system is not presenting a lot of reflected power back to the output of your radio. A bad antenna with a poor impedance match will present beautifully to your radio if you lengthen the cable enough.

 

Hope this helps.

 

 

Michael

WRHS965

KE8PLM

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SWR versus return loss or reflected power.

 

People get obsessed with 1.2:1 or very low SWR but it's not that big of a deal.

A  1.9:1  SWR is a 10dB return loss meaning that 10% of your power is reflected back to the radio.

This is not that bad.... and not that rare.

In my book anything 2:1 or better is pretty decent.

 

Even a 3:1 SWR is a 6dB return loss meaning that 25% of your power is reflected back to the radio.

It does not mean that this power is completely dissipated in the finals of the radio either.

Radio outputs are not often that close to a 50 ohm Z. So some of that gets re-reflected.

Transmitters are designed to work into a 50 Ohm load.  That does not mean they look like 50 Ohms.

 

Of course for a 10KW radio station it's a different matter.

These numbers get kind of big.

 

Vince

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SWR versus return loss or reflected power.

 

People get obsessed with 1.2:1 or very low SWR but it's not that big of a deal.

A  1.9:1  SWR is a 10dB return loss meaning that 10% of your power is reflected back to the radio.

This is not that bad.... and not that rare.

In my book anything 2:1 or better is pretty decent.

 

Even a 3:1 SWR is a 6dB return loss meaning that 25% of your power is reflected back to the radio.

It does not mean that this power is completely dissipated in the finals of the radio either.

Radio outputs are not often that close to a 50 ohm Z. So some of that gets re-reflected.

Transmitters are designed to work into a 50 Ohm load.  That does not mean they look like 50 Ohms.

 

Of course for a 10KW radio station it's a different matter.

These numbers get kind of big.

 

Vince

 

I agree. If you're under 2:1 - you're in the right neighborhood. If you can get it down to 1.5:1 without much work - you're good.  You can go lower than 1.5:1 with VSWR, but it's only worth the effort if it's really no effort at all as it's not worth investing a lot of time into.

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I like the way you think... I ordered one of these to test already and it should be here today.

 

 

I have one of my antennas mounted on the center inside edge of the trunk on my Toyota Camry. Without this multi-axis mount the antenna would not be vertical.  ;)

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I have one of my antennas mounted on the center inside edge of the trunk on my Toyota Camry. Without this multi-axis mount the antenna would not be vertical.  ;)

 

I meant to reply back to this.  The mount I got is an absolute "no go".  Because it's so wide (about 4") it won't fit on my car because the only available edge is curved. :/

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Might have to go with a hood mount by the windshield with a s much antenna as you can get to clear the windshield and roof.

 

Outside of that you may be stuck with a mag mount.

 

Depending on the shape of the trunk a lip mount on the side of the trunk. Similar to the link below.

https://hamradioschool.com/wp-content/uploads/2014/07/GetStarted-bands.jpg

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i stumbled on this info a bit ago via reddit, though at the time it was limited to the graphic.  haven't been able to find what frequencies were involved in the testing, as the link to the catalog article is now dead.  i don't feel as bad about running mag mounts (not too worried about my paint anyway), though with the last puchase i DID go out of my way to find one with better (rg8x) cable.

 

i will also concede this is just looking at losses in general, and not effect the different placements have on the radiation pattern.

 

https://kv5r.com/ham-radio/mobile-antenna-placement/

 

mobileant.jpg

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I have a MXT275 and got a Midland 6 dB Gain Antenna & Midland Antenna Mag Mount that has a rubber base and mounted it center front of my roof on my 4Runner. Seems to work and though it looks like i can tune the antenna im guessing its fine. 

 

Apparently i can talk to people and they can talk back to me so i guess its fine. lol 

 

But i wish i knew how good bad it is. 

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