Tune magnetic mount antenna prior to install or wait until coax is ran and then tune?

[Lo/Dx Frontier]

Will it make a difference setting the SWR on a magnetic antenna before I run the coax through the vehicle? I’d like to tune it and see how well it works before I go to the trouble of pulling panels and running the cable.

 

[Greg T]

Always tune after full install.

 

[DoctorAudio]

If you have a large roof, don't put it dead center or it may not tune. Even on a PU roof you can center it right to left but front to back, place it closer to the front or back edge. A quarter wave antenna wants to see some sloping ground plane close to the feedpoint.

 

[Lo/Dx Frontier]

If you have a large roof, don't put it dead center or it may not tune. Even on a PU roof you can center it right to left but front to back, place it closer to the front or back edge. A quarter wave antenna wants to see some sloping ground plane close to the feedpoint.

I wish I’d seen this before I cut on the whip to tune it. I had it dead center. I just now moved it about a foot towards the rear. I’ll check SWR again and see how it has improved. Yesterday I experimented with it moved back even further and with no other changes I saw the SWR drop a 1/4 on channels 1 and 40, but I had put it back center again after that. I moved it again just now simply due to thinking it looked better farther back. I think it looked a little bit silly sticking up right in the center, like a propeller on a beanie cap, LOL. I’ll post a before and after pic below.

BEFORE


AFTER

 

[M0GVZ]

If you have a large roof, don't put it dead center or it may not tune. Even on a PU roof you can center it right to left but front to back, place it closer to the front or back edge. A quarter wave antenna wants to see some sloping ground plane close to the feedpoint.

Sorry but this is just plain wrong. The only difference between changing from horizontal and sloping is all other things being equal it changes the feedpoint impedance so you need to retune the antenna.

The reason it won't tune is because if you have a large metal roof you have got an efficient ground plane and CB mobile antenna whip lengths are made assuming a poor one as most installs are. The more efficient the install the shorter the length of the antenna needs to be for a given frequency. Have a really efficient one and there's not enough adjustment in the antenna so you end up having to cut it.

 

[DoctorAudio]

If you are going to claim what I said is plain wrong you are going to need more than a rambling response to bolster your standpoint.

1/4 Wave Ground Plane Antenna Calculator – M0UKD – Amateur Radio Blog

"A quarter wave monopole mounted against a perfect ground will have an impedance of around 36Ω but by bending the radials down at an angle of 45°, we increase this to around 50Ω."

No amount of radiator trimming will get you to 50 ohms without at least some of the ground plane sloping close to the feedpoint.

 

[The DB]

If you have a large roof, don't put it dead center or it may not tune.

Is it possible that this is the case? Yes. So the next questions becomes...

How large of a roof would it take for something like this to happen? And then... Are their any vehicles that are actually that large?

The smallest size would this would be possible would be be a roof that is larger than an electrical 1/2 wavelength from corner to opposite corner. The reason this comes to mind is the widespread use of radials and how they work... Think the distances to the corners are radials. It would need to be at least that big to get past the 1/4 wavelength optimum length from the antenna, would it not?

Further, I'm not convinced that going larger than this will necessarily have a negative effect. Working with a sheet of metal as a ground plane is very different from working with radials. Sheets of metal as ground planes tend to be far more forgiving in my experience. While I have not worked on vehicles that large, I have, occasionally, worked on antennas on metal roofs of buildings which are much larger than this. I didn't have any issue getting said antennas (which were mounted either in the center or very close to it) tuned...

So, I see there is a possibility of what you are saying being true, however, even if it is, I don't see it coming up for the CB band unless you are using something like a very large mobile home with a metal roof, and even then I would still question if such a thing were necessary, or beneficial, or if you would even notice the difference...


The DB

 

[brandon7861]

If that antenna mount was easily disassembled and coil form removed (like in VHF MaxRad NMO mount antennas), I would remove that and connect the nano to the coax and port extend to the infinite impedance point (so I knew my measurement plane was at the loading coil connection) and then I would short across the coil with foil and reinstall it so it was a direct connection to the whip and measure the impedance of the whip alone bypassing the coil. With the whip impedance known, the coil can be mathematically solved (and changed if needed).

 

[brandon7861]

adding to my last post instead of editing it.

While the whip is directly connected and coil shorted across, the whip should be adjusted to where it meets the 50Ω constant resistance circle. The coil (if only in series with the whip) is just canceling capacitive reactance, so the resistance needs to be right from the start. If that is not possible (ie. the resistive portion is lower than 50Ω), it may be necessary to configure the coil so that the bottom end is grounded and the coax goes to a tap point. Doing that allows you to match any impedance in the lower half of the smith chart that is to the right of the constant conductance circle going through the center of the chart

 

[Handy Andy]

To help, draw an outline of your truck...

Preferably from the side...



OR even a VAN ...


Both are moving Ground Planes - with a View - (Steering optional but preferred)

Any mag mount antenna - per this threads discussion - if laid flat perpendicular on any counterpoise surface - the impdance is 36Ω approximately

So when you get closer to the edges, what is happening?

The counterpoise provides a "slope angle" for the signal - so coupling for that impedance point does change it's "appearance" to your SWR meter.

So, that "Slope" or drop off - does affect the SWR.

This is a crude demonstration but I hope you can visually observe the changes in the "terrain" the antenna works in, will in some way affect the SWR at the meter - by the changing of impedance the antenna appears as, at that particular spot on the vehicle.

So you can match the antenna to a 50Ω known impedance - BEFORE you put it on, but does not guarantee the 50Ω appearance when it's on the vehicle.

Location plays a big role in how this changes the ballgame.

 

[9C1Driver]

Will it make a difference setting the SWR on a magnetic antenna before I run the coax through the vehicle? I’d like to tune it and see how well it works before I go to the trouble of pulling panels and running the cable.

I've done that with every new company car I get over the years. You can slap the antenna up their and check the SWR. If need be move it around and see where it works best. Once your happy just testing it in your driveway then do the full install. Once your done re-check the SWR, as long as the mag base is in the same spot it should be good to go.