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Antenna Choices & Calculations


tweiss3
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I had originally planed to use an X300NA antenna to put up a repeater AND packet services for VHF ham. Today, I finally got around to unboxing the X300 and assembled. I clamped it to a tripod mount 5' above the ground in the middle of my driveway to check how it swept with the VNA. I also have an antenna that came with the repeater, that I think is a Laird Technologies FG4603 based on the size, color and shape. The specs between the two are below:

X300 - 10' tall, 9.0db gain on UHF, 5/8 wave five element

SWR: 462.68MHz - 2.525:1, 467.62MHz - 3.125:1, 449.67MHz - 1.461:1, 438.36MHz - 1.237:1

 

Laird - 3.66' tall, 5.0db gain

SWR: 462.68MHz - 1.730:1, 462.30MHz - 1.698:1, 467.62MHz - 2.150:1

 

With the above, I have 22.8W leaving the duplexer, and with the mixture of fittings and coaxial, my coax loss is 0.559db. 

Using an SWR power forward calculator, and a EIRP power calculator I calculated the following:

X300: 19.1 W forward power gives 53.40dB EIRP

Laird: 21.28 W forward power gives 47.87dB EIRP

 

Leaving everything the same, mounting height, coax, everything, and knowing here is a 6.34' top of antenna height advantage and 5.53dB EIRP advantage for the X300 with higher SWR over the other antenna, which one would you choose and why?

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@WROZ250  Did you look at my images?  I tested the X300 on a tripod at 8' with a 3 foot patch cable before I put it on the roof.  It was 1.6:1 or better on every GMRS channel.  30 feet off the ground with a ground strap, its even better.  The X300 has much wider coverage on UHF than VHF.

 

I will say that if @tweiss3 truly has a 2.5:1 SWR on the 462 frequencies, I would say something is wrong, other than antenna choice.

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49 minutes ago, Lscott said:

What is the goal? You want one antenna for Ham and GMRS or just one of the two?

Intent is to put up a repeater on 22 462/7.725

I'd like to setup a winlink gateway on VHF, but if needed I can figure that out later.

This antenna is loved away from my other antennas, and is for services only, not normal use (Winlink, iGate & Repeater).

My only must right now is the repeater.

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The high gain antenna would be nice BUT the match sucks. That’s leaves the lower gain one. Since this is for a dedicated application, repeater, I wouldn’t worry about the SWR on the 467.xxx frequencies since these are all inputs, you’re not TX’ing on them. The lower SWR on the 462.xxx frequencies is what you need to be watching since those are the repeater outputs.

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3 hours ago, gortex2 said:

Personally I'd use neither. If you want a repeater I'd order a commercial LMR UHF antenna. the DB404 is a solid performer on GMRS.

The Laird is a commercial antenna designed for that frequency range, with the golden metal tip and golden base. It came with the repeater.

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I'm courious about the SWR on the x300.  I have the x300 repeater antenna mounted at my house.  The extra height and gain contribute to extending the coverage area. I got an additional 5 miles but switching from the 200 to the 300.

 

I'm going to go grab my analyzer and checking my SWR.  You my have something wrong on your end.  Last I checked, my SWR was 1.5 or 1.6.

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The X300 is a ham band antenna, hence the higher vswr on GMRS. 

So for whatever additional 'gain' the X300 has compared to the Laird, you're losing energy due to the higher vswr, which is power that isn't being radiated.  Stick with the Laird, at least most of the power being sent to it is actually going out over the air.  The other thing with the X300, is that because it would be operating out of it's designed frequency range, the antenna pattern may (may) be different than it would be when operated in the ham band.

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The laird is not what I call a repeater antenna. I wouldn't install it on a tower. Those are used primarily in the LMR world for control stations and similar uses. I guess if thats all you have thats what I would use over the HAM antenna. 

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On 1/22/2022 at 10:47 AM, marcspaz said:

@WROZ250  Did you look at my images?  I tested the X300 on a tripod at 8' with a 3 foot patch cable before I put it on the roof.  It was 1.6:1 or better on every GMRS channel.  30 feet off the ground with a ground strap, its even better.  The X300 has much wider coverage on UHF than VHF.

 

I will say that if @tweiss3 truly has a 2.5:1 SWR on the 462 frequencies, I would say something is wrong, other than antenna choice.

Sorry marcspaz, there were so many responses on this I never saw your pictures (I will look though).

I was responding to tweiss3's post and I wasn't surprised by what he posted and, took those comments on face value. 

While I have no direct experience with the X300, I do have the X50NA which does not work well out of ham band.  Indeed, at UHF, the ham band performance it isn't really what it should be IMHO.  At UHF in general, the vswr of the X50NA (as viewed on 2 different analyzers) seems to waver up and down every few MHz between ~1.4 and 2.5 vswr and the average of that wavering only goes up with frequency.  Dual band antennas always seem to be a compromise to some extent, so again, I wasn't surprise at tweiss3's comments.  Also, and perhaps to your point, environment (where and how it is installed) can affect antenna performance and of course, there could indeed be something wrong. 

All that said, I do agree with your comments about tweiss3's situation.  Assuming of course that your X300 is typical of that model's performance.  As noted above, tweiss3's situation could be caused by where and how the antenna is installed.

FWIW, I do use the X50NA for listening to GMRS and while it's only about 20ft AGL, the reception is pretty decent even with the bad vswr, as I can hear repeaters between 90-100 miles away with pretty decent signal strength.  This surprised me as my location is surrounded by various mountains which are between my location and the aforementioned repeaters.  I can, and have, transmitted on GMRS with the X50NA and it does work and, while I really don't think it's going to 'blow my transmitter up' I just don't like operating into an antenna system with a high vswr.

🙂

    

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@WROZ250... I appreciate the detailed response. No arguments here. 

I have to be honest, this is my first x300 and I don't know if this is normal/typical performance. I assumed it was since it's a stacked array with induction coils and a capacitance hat inside the tube to give e it flexibility and hight gain.  I do find it odd that it's both wide in range and high gain.  Normally I've seen a sacrifice in bandwidth as gain increased. 

I use to use an x50. It was fine for portable ops (which is what I used it for) but wasn't great.  I went with an x200 for portable and an x300 for the house... mostly because I an switching the x300 between 3 different radios and seemed to have the best performance where I expected to use it.

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On 1/22/2022 at 11:51 AM, marcspaz said:

Here are the readings on my X300 with a 30' run of LMR400.  Not getting on the roof anytime soon.

 

Did you add a ground strap to the base of the antenna?  I have a run of 6 gauge going to a ground rod.  May be the difference. 

This was a test setup just to measure where it landed in GMRS, knowing it was a ham antenna. I think you have some serious masking in your feedline, because my measurements aligned directly with the claimed SWR for the ham band.

image.png.16ef71fabb8d260fd1069277e2f12f95.png

My hope was that outside of the band, it wasn't too steep and would work, but it is a bit too far out of band.

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3 hours ago, tweiss3 said:

I think you have some serious masking in your feedline, because my measurements aligned directly with the claimed SWR for the ham band.

 

I can't test mine against the chart above anytime soon.  I am disabled and can't get on the roof.  I have to hire a climber.  During my original test, when I measured the 1.5 or 1.6 (don't recall exactly), it was on a tripod with a 3 foot patch cable.  The feed line should not be an issue in that case. 

If we calculate for the masking of 100 feet of the LMR400 I am testing on now, with the 1.17:1  I am measuring, that would still only be 1.4:1 unmasked VSWR, which is still good and agrees (close enough) to what I measured while it was on the ground.

 

Not sure what to say.  My x200 is in the garage and close in performance.  While its not the same antenna, it may shed some light for me.  I may test it today or tomorrow while on the tripod.

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20 hours ago, WROZ250 said:

At UHF in general, the vswr of the X50NA (as viewed on 2 different analyzers) seems to waver up and down every few MHz between ~1.4 and 2.5 vswr and the average of that wavering only goes up with frequency.

I've seen this with a number of high gain antennas on UHF, at least mobile ones I've tested. I attribute this to the different sections of the antenna with the associated phasing elements between them. If you're lucky the valley of the SWR ripple is where you want to operate on GMRS. Otherwise you could end up near the peak. It all depends on the particular antenna and sometimes it's mounting location sensitive, close to the ground or elevated some 10's of feet up on a mast.

While some antennas are marketed as ground independent they still show some effect, an improvement, when a small ground plane is present. There are some people who added a small set of ground radials at the base, of the antenna, when mast mounted and claim it made a noticeable improvement in the match. The ground radials seem to help keep RF off the exterior of the coax shield which shows up as a higher SWR at the radio end.

Some antennas like the J-Pole/SlimJim types have a ferrite installed on the coax at the feed point, or a loop of several turns of coax, which is necessary to keep RF off the coax shield.

I've also seen mobile magnet mounts, like a CB antenna, where the SWR changes when the coax is moved around, not the antenna. The antenna doesn't couple into the metal very well through the base because it's too small for good low impedance capacitive path to ground so some of the RF ends up on the outside of the coax shield. I even had this problem with a 6M 1/4 wave magnet mount on a van.

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Well, I installed the commercial antenna I had just above the roof line because the wife said I needed to clean up my office, and 1-1/4" feedline and parts were all over the place. I have a few months to maybe find a better antenna deal, but for now, I'd say that X300NA is not good for GMRS. If I could find two folded dipole arrays, I'd combine them onto a single mast, or even if I could find a DB-315 (no longer in production, super hard to find). Not looking to spend huge money, as I'm still going to be limited on height.

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