Sirio MAG 145 PL mag mount

[The DB]

@Dr_DX, you went beyond your own words in an effort to make your point. I always give props to people who post more than just their thoughts.

Two things.

One, something no one on this forum will argue with you on, yes, an electrical half wavelength of feed line will repeat the impedance of the feed point of the antenna. This seems to be something you are trying to push really hard, and I will tell you this right now, no who posted in this thread disagrees with this statement, so at the point all you are doing is preaching to the quire.

Two, you make the assumption that because of the above, that the electrical half wavelength point is the only accurate point you can use to measure SWR. Honestly, I get it, its an easy mistake to make, but it is still a mistake. Nothing you linked or posted said or implied this in any way. You can put all the evidence together that you want to say that an electrical half wavelength of feed line repeat the antenna's impedance, but this tells you absolutely nothing about how SWR is affected by these changes.

Look, if you want to use an electrical half wavelength of coax and use SWR as its measurement, by all means, go for it. At the same time, if someone wants to use exactly 18 feet of coax for the same reason, good for them. I would say to use the shortest length of coax to get from the antenna to the radio, then add a few feet for if you need to move the radio to some degree, that is even better. They should all show the same reading, and if they don't, then guess what, you have a problem that using an electrical half wavelength of coax is neither going to fix nor/hide, and strangely the 18 foot length will fix/hide it far better... Go figure.

I highly recommend reading "Another Look at Reflections", and if you have read it, I recommend reading it again. The Reflections series of books by the same author is good as well. M. Walter Maxwell, who you quoted above, disagrees with the premise that you need an electrical half wavelength of coax for anything related to SWR measurements, and he actually talks about why.


The DB

 

[Smokinone]

An observation. The Sirio 145 Pl, when attached to the MFJ269, shows about X or Xs of 12. It's length is 16.4'.
The Wilson trunk mount, when attached to the MFJ, shows X or Xs of 0. It's length is 17' 5 1/2"

If "X" is what we're looking for the longer Wilson coax looks to be length for RG58 au/cu

27.400 freq

Both are RG58, 1 au and 1 cu

Just trying to figure all this out.

Thanks

 

[M0GVZ]

LOL A lot of very well known Hams say otherwise...

None of them worth listening to when it comes to antenna systems if that's what they're saying. Whilst there are situations where it's necessary to have coax of specific lengths or you can take advantage of the characteristics of coax being specific lengths a single end fed monopole mobile antenna sat on a vehicle is not it.

In 99.9% of cases the correct length of coax to use is the shortest amount you need to get from radio to antenna by the route you want it to take.

 

[M0GVZ]

An observation. The Sirio 145 Pl, when attached to the MFJ269, shows about X or Xs of 12. It's length is 16.4'.
The Wilson trunk mount, when attached to the MFJ, shows X or Xs of 0. It's length is 17' 5 1/2"

Just trying to figure all this out.

Thanks

The reason for the difference is the fact that the magmount is using capacitive coupling to ground, the trunk mount isn't. When you're using capacitive coupling you'll never achieve X=0 because there will always be some element of reactance.

 

[Smokinone]

The reason for the difference is the fact that the magmount is using capacitive coupling to ground, the trunk mount isn't. When you're using capacitive coupling you'll never achieve X=0 because there will always be some element of reactance.

That makes sense. The Wilson 1000 tuned fine with the trunk mount and also with a roof mount application. 2 cars with several different locations with the mag mount on the vehicle, the 1000 doesn't seem to be able to tune. It appears the whip is too short although they are original whips for the antenna's. I have 2 and tried both with about the same results. I could probably use a trunk mount on one of the cars, but I really wanted a quick and easy...if that exists...way to put the antenna on and off quickly. Also the trunk mount Wilsons have been discontinued.
Just trying to use what I have as well.

Thanks

 

[Smokinone]

It seems I skipped an important step with the Mag Mount. It appears that there is a foil sticker that goes on the base of the mount. I missed this initially, but I never thought I would have to read instructions for a mag mount...wrong. At any rate, after attaching the foil sticker to the bottom, I re-tested the mount with the Wilson 1000 and got 1.4 to 1.1, low to high.
As it stands, I will leave the coax that came with it on and maybe down the line change out to RG400.

Thanks.

 

[ForestRunner98]

The foil decal was it!!! That’s awesome!!!

 

[Smokinone]

The foil decal was it!!! That’s awesome!!!

Strange but true.

 

[Groundhog KSS-2012]

An observation. The Sirio 145 Pl, when attached to the MFJ269, shows about X or Xs of 12. It's length is 16.4'.
The Wilson trunk mount, when attached to the MFJ, shows X or Xs of 0. It's length is 17' 5 1/2"

If "X" is what we're looking for the longer Wilson coax looks to be length for RG58 au/cu

27.400 freq

Both are RG58, 1 au and 1 cu

Just trying to figure all this out.

Thanks

I attempted the calculation needed and IF, I'm right, the exact 1/2 wave length of RG58 is 11.88 ft.
I may be way off but, this is the number I came up with using these calculations.

RadioMods coax length page

A page which shows how to work out the correct coax length.

www.radiomods.co.nz

Again, I could be way off. If I am I'm sure someone will let me know. Have a great day everyone.

 

[EI7KN]

I used to swear by the Sirio 145 and now I swear AT the Sirio 145 Mag Mount. It is imperative that the supplied aluminium foil is attached to the bottom of the mag mount and then the rubber boot is put over it. This mag mount will not work properly without it. This is the only mag mount that I have ever found which cleverly makes an exceedingly good capacitive connection between the mag mount and the vehicle. This means that when it works, it is not necessary to have any additional grounding of the aerial, to the vehicle's chassis. This means that the mag mount can be used for HF bands down to 40m (7Mhz) with ease.

It is also essential that when dismantling and reassembling the mag mount that care is taken to ensure that the tag is connected to the same brass pillar as originally assembled, as that is the only one which is internally connected electrically to the applied tin foil. It's therefore very worthwhile to mark the orientation of the coax to the bottom part of the mag mount body before disassembly.

The problems with the Sirio 145 are fixable but are a result of poor design. Usually there is insufficient glue and any tug on the coax will likely disconnect the coax from the SO239's centre socket. I've found that the application of a tie-grip pulled very tight inside the mag mount and around the rubber grommet tube and with the excess snipped off, followed by hot glue from a decent hot glue gun, will provide superior strain relief. However before doing this, there are some other areas which need addressing.

The bottom of the tag, to which the coax braid is soldered to, needs to be clean. Do NOT rough this up with sandpaper or a screwdriver, as this will result in less surface area for conductivity. Instead use something like a Dremmel, fitted with a SOFT polishing disc. Also gently polish the head of the brass pillar which is electrically connected to the aluminium foil. Do NOT NOT NOT apply WD-40 as this acts as an insulator and not as a conductor!

The aforementioned tag is connected to a ring which is attached to the SO239 by compression. Ideally you will have some decent flux, leaded solder and a HOT soldering iron (480C). Make sure before you start soldering that the tag is lined-up with the bolt hole. Apply solder joints around the circumference of the join between the SO239 and the ring. A dozen or so joints should do it. Clean the finished joint and remaining flux with Isopropyl Alcohol.

In an ideal world Sirio should have used a decent SO239 which has solder joints to both the inner and outer, rather than using a nasty and ineffective compression fitting. Likewise a solder joint to the pillar which connects to the aluminium foil would have made for a much more reliable product.

Out of the box the Sirio 145 works well (providing that the aluminium foil has been attached), but the unit fails with use and age because of using crude contact rather than soldered connections.

Changing the coax and careful length measurement is not required, though the mag mount needs to be in good order and the antenna needs to be correctly adjusted for VSWR. Once again, the tin foil supplied MUST be installed for this particular Sirio mag mount model. This is the only SIRIO model which is supplied with and NEEDS this component.

 

[M0GVZ]

It is also essential that when dismantling and reassembling the mag mount that care is taken to ensure that the tag is connected to the same brass pillar as originally assembled, as that is the only one which is internally connected electrically to the applied tin foil. It's therefore very worthwhile to mark the orientation of the coax to the bottom part of the mag mount body before disassembly.

I'm not sure whether to hug you or hit you for bringing this back up .

Seeing that post made me paranoid I'd not checked when I fixed mine years ago when the braid came desoldered so I just took it apart to check. Good job I did because the rubber waterproof sleeve around the coax where it goes into the base had come out, I'd not hot glued the coax in and the center conductor was just about to part company with the center pin on the SO239.

Also that brass pillar doesn't connect to the aluminium on the bottom of the mount, it connects to a flat metal disc in the bottom of the mount. If you look at it too the brass pillar the tab is meant to go to is a different shape to the other three. It is round whilst the other three are hexagonal shaped.

 

[EI7KN]

The flat metal disc is what connects to the foil, or am I mistaken? I do not have a Sirio 145 without the metal foil attached, so I can't check. Thank you on the correction about the round rather than hexagon shaped pillar, that's an absolutely cracking spot

I'm going to order a brand new unit and make a video of the unboxing and the changes required to make this a better performing device. No doubt folks will criticise my soldering, but I for one wouldn't blame them!

 

[M0GVZ]

No, the flat metal disc doesn't connect to the foil, well not directly. You can however check for continuity by making a very small hole in the center of the foil, putting a probe there's another metal disc there that's attached to the one you can see when you remove the top section that I guess retains the magnet?

As the foil sheet is glued to the base there's no electrical conductivity on mine at least, the glue is basically an insulator.

It definitely did something doing the repairs as it altered the tuning slightly, a couple of hundred kHz, and from memory I'm sure that the SWR curve on my Sirio 5000 is a bit sharper now so I guess I did have an issue.

It's a shame they have these issues with the coax securing because other than that they're the most solid single magnet magmount I've seen. I've gone under drive-thru barriers with my Sirio 5000 on, the antenna bent 90 degrees and the magmount hasn't moved. I think only triples would be any better.