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Mobile 102" whip tunes great, but other antennas won't

Discussion in 'CB Antennas' started by buickid, Jul 8, 2020.

  1. buickid

    buickid Active Member

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    Helping a friend with his mobile. Vehicle is a Ram Promaster van, big roof, mount looks well installed. He has some sort of competition-style open coil antenna on a roof mounted puck. No matter how we adjust the stinger, we can't get SWR on various meters or my MFJ-259B below 1.8-1.9 on 1 and 40, with a slight dip (1.8 or so) in the middle. I suggested trying my 102" whip with a ~5" spring for kicks. On the MFJ, it dropped right down to 1.2~1.3 on 1 and 40, and about 1.2 in the middle, fantastic. Since the van is so tall already, he wants to run his coil antenna to shave off a couple feet, but the SWR is a bit much. We tried a Predator 10K I had lying around, same result as his coil antenna, broad but high SWR. A Firestik had the same result.

    Any idea why the 102" whip tunes up just fine but others won't? I have a feeling it has something to do with the 102" + spring being 1/4 WL, but I can't explain it. He thinks the coax length, I've tried to explain the fallacy of tuning via coax, but without being able to explain anything else, he's having a hard time believing it.



    Thanks for your help!
     

  2. secret squirrel

    secret squirrel Lustrous Potentate

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    Is the RAM Promaster an all metal roof or does it have the fiberglass roof extension like an ambulance? Since you have the antenna analyzer go ahead and try the 18 or 21 feet and see if it makes a difference, you will be able to see the results and resonance. if there is the extended roof then you will have a ground plane issue. I work as a paramedic any place that composites can be used to reduce weight on the new style vans they have used it. The radiation pattern of the 102 inch whip may be helping you results. The land mobile antenna for VHF that are used on ambulances are base load with hood channel mounts, most stay away from roof for clearance issues. If the roof is used its the short 18 inch 1/4 wave whip.
     
  3. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    buckid, I can only go by what my modeling shows, but my mobile models that have large horizontal roof lines do not match or perform as well as a pickup trucks model with a much smaller roof line.

    So, even if I can get a model with a large roof to show a workable match...it does not show as good of performance either compared to a model like a Pickup Truck with 2 doors. I realize this flies in the face of what we often see represented in mobile shoot-out videos.

    That said, my only conclusion so far suggest that the larger flat surface mobiles under a 1/4 wave antenna is showing a similar matching response like I might get with a 1/4 ground plane antenna that has 4 or more horizontal radials, and is close to the Earth like a mobile setup.

    Let me see if I can find some models that may help demonstrate what I'm talking about.
     
    #3 Marconi, Jul 8, 2020
    Last edited: Jul 8, 2020
  4. M0GVZ

    M0GVZ Sr. Member

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    When you were using the MFJ 259 and analysing his antenna how much either side of the 40 channels did you go? You need to sweep at least 1-2MHz below and above. If he's really got a very efficient ground it may be that at it's factory length it's actually resonant around 25-26MHz, way below Ch1. Quarter wave antennas are very broad banded so can show a low SWR over a few MHz which would be the reason why you could see a low SWR on your 102" whip but not on the two antennas you tried.

    On my last vehicle I used a fixed mount on the roof, had a very good RF ground, spent lots of time doing bonding ala K0BG's advice and with the antennas I used to use they'd end up resonant just below 26MHz shortened as much as they'd go and it would be a case of having to get the dremel out and cut off 2-3".
     
  5. buickid

    buickid Active Member

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    Thanks for the reply! As far as I can tell, the whole roof is steel. We've tried different jumper lengths on the MFJ with similar results.

    Thanks for replying, always appreciate your posts.
    Interesting about the modeling of larger roofs. I look forward to the models that may help explain this phenomenon.

    That could be something. I'll have to try a wider sweep. I didn't go much past 26.965 and 27.405. The rooftop is huge but it's also quite high, so maybe that has something to do with it. He's adding some bonding, so we'll see how that affects it as well. Will report back with the results of a wider sweep next time I link up with him.
     
    Slowmover likes this.
  6. buickid

    buickid Active Member

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    Got a chance to sweep it today, here we're the results:

    26.0 3.8
    26.2 3.5
    26.4 3.3
    26.6 3.0
    26.8 2.8
    27.0 2.6
    27.2 2.3
    27.4 2.2
    27.6 2.2
    27.8 2.2
    28.0 2.1
    28.2 2.2
    28.4 2.2
    28.6 2.3
    28.8 2.5
    29.0 2.7
    29.2 3.0
     
  7. G GOLLY WAlly

    G GOLLY WAlly WDX 719 / Waldo on AM

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    I have the same problem with other antennas, can't get them tuned where I like, but my 102 whip tunes great every time, that's why I usually qo with the 102.
     
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  8. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Buckid, I made my model in the PDF file below. It is my crude idea of the dimensions for your Van with a 107" whip...like you told us in your post #1 above...where you saw 1.20:1 SWR.

    Also here is the bandwidth curve I got with my model when I set the whip to 106.8" inches to resonance. My match shows R = 38.02 ohms. and x shows +0.07552 ohms = 1.315 SWR.
    Here is the antenna pattern too, showing - 2.59 dbi gain at 16* degrees above the horizon. This gain is less that 1 dbi gain and it typical for HF mobile antennas with some omni directional gain.

    I think this looks very close to what you have reported above.

    I will post my other model that shows improved gain after you've had a chance to look over this model first.

    I have to give some credit to The DB for his idea for a better mobile model that he posted a while back. but I could not find DB's old work. So, I did an Internet Search for Modeling a Mobile Antenna, and found an article on Eznec Modeling for a Mobile Antenna by Jim Andrews, KH6HTV from 2005. Jim gave me the details i needed.

    https://kh6htv.files.wordpress.com/2011/09/eznecmobileant.pdf
     

    Attached Files:

    #8 Marconi, Jul 11, 2020
    Last edited: Jul 11, 2020
  9. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Pretty much what I would expect that any mobile antenna (below 50 Mhz) would be a LOSS. Plus the lower in frequency the greater the loss.
    So retro speaking over a dipole at the same height...a loss of almost 5dbi (since a Dipole under worse conditions is +2.2 dbi)
    So a nice model!(y)
    All the Best
    Gary
     
    Slowmover likes this.
  10. buickid

    buickid Active Member

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    Hey everyone, sorry for the lack of updates, been busy having just changed jobs.

    We did a little experimenting in a park the other day. We took the antenna off his puck, and tried different placements of a Wilson 5000 magmount on the roof. First, a little about the van roof. It looks like the majority of the center of the roof behind the front two seats is one big rectangular panel which is surrounded by: a panel above the front seats, narrow panels (six inches wide or so, and rounded) running the length of the left and right sides, as well as a similar sized panel across the back. They are connected with spot welds and seam sealer.

    When we put the mag mount on any other panel beside the center, SWRs were reasonable. 1.5-1.6 with a healthy dip in the middle. The curve may have been slightly out of the band, but nothing I wouldn't expect to be able to tune into the band with antenna length. Even if you put the mag mount right on the edge of panel near to the center panel, SWR just fine. As soon as you put the antenna on the center panel, boom, bad SWR.

    I'm thinking the panels are not well connected together with the minimal spot welds and creating some sort of capacitance or something between the center panel and surrounding sections. I've recommended to my friend that he figure out a way to bond that center section to the outer sections. He's going to try some metal flashing or braid, clamped across the seam with magnets, paint ground off underneath. We'll see how that goes!
     
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  11. Slowmover

    Slowmover Elmer

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    That was my thought on reading the thread. Check operators manual to see what it says about roof loads. Can you stand on it? Racks mounted to roof or actually to ribs AT vertical wall juncture? Etc.

    The roof may not be a contributing structural member; but more like a tie. (Least weight & easiest manufacture).

    I’d go to bare metal at every corner and at a reasonable distance between with RF bonds.

    I’d also get underneath. Unit-body vehicles may use rubber-isolated sub-frames for drivetrain. (For all I know, the entire body can be lifted off those subframes w/running gear).

    Uni-body is strong enough as an open container (no roof) on something like a van that this wouldn’t be surprising.

    Last, fleet sales. Every region in the country and every major city has a vehicle-brand specialist fleet dealer. They know more than the average dealership. Biz is government and corporate fleets.

    Also see: eLightbar (com).
     
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  12. Mustang 131

    Mustang 131 Sr. Member

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    All vans behave like tha, if your running one antenna. The sweet spot is right near the back edge every time.
     
  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    buickid, I also think that with modern mobile construction for Pickups and Cars are built to help make them quieter on the road, and that may be a problem in disguise.

    A while back I was piddling around with your idea for a large Van with a very large flat roof. I made 2 new mobile models of the basic design of a very large Van. I posted the 1st model to represent your idea.

    However, I got to thinking that some members might look at such a model and the details...and their eyes would cross...saying what is that? :rolleyes::rolleyes::rolleyes:

    I think maybe you're on the right track with some good bonding and I encourage you to continue to a solution.

    Let us know how your project with your buddy turns out, and try and compare his new setup with your mobile or some of your local buddies, OK?
     
    #13 Marconi, Jul 28, 2020
    Last edited: Jul 28, 2020
  14. buickid

    buickid Active Member

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    So, delayed update to this saga. The buddy tried doing a little bonding with some copper flashing held down to both sides of the "joints" of the roof with magnets. He sanded down to bare metal under those spots. He applied this to maybe 3-4 spots. It did help the SWR a bit. I encouraged him to continue bonding the vehicle. He's turned to adjusting the coax length and changing antennas to get the SWR down, even though I explained to him that coax length doesn't do anything for the antenna performance. Some very vocal people swear by the coax length, so I guess that won out. We'll see, maybe he'll come around and do more bonding. I really wanted to see if we could optimize the van.
     
    G GOLLY WAlly and Slowmover like this.
  15. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    In my experiences with vans, many of them get "conversions" done to them.

    And yes, @buickid - the Roof is the weakest link. Because of the very reason of mounting - it also uses the smallest surface area - some late model vans are simple "crimp" and glue, not necessarily weld, due to the fact of rust inhibitor and epoxy/adhesives used to seal the roof from water entry. It's a stronger bond - believe it or don't, than the spot welding used in earlier ones, that are easily seen as "running rust stains" at drip lines or even along the seams.

    If you have time you may also discover that although there is a "ground" connection up there, it is done thru wiring and not at the "skin" or surface finish metal - it's on the pillar - if any. So the "bracing" those ribs that go across from side to side windows, are your actual frame ground - the skin metal above is single layer and pretty thin gauge #AWG but ribbed in itself to withstand torque and provide some rigidity to the otherwise pretty solid framework it rests on.

    To try and "bond it"? You're better off using Roof rail and/or rack rail system to provide the length and rigidity needed to handle such a beast for the Roof needs the Pillar more than the Pillar needs the roof, if you follow my meaning. The Roof is the only part of the vehicle they only provide some ability to haul stuff but little else in protection from overhead elements.

    There's simply not enough to "just barely" enough metal" (Transits are the worst offenders - some are now completely plastic - think "Saturn")

    So the "drop off" feedpoint or raising the feedpoint slope (height of radiator to skin) does make a difference so others may say otherwise the location is paramount and its' level of angle slope away from the point it's mounted to or at - sloping away and down makes a difference more than "coax length" but who am I - I just post around here...

    Good luck and keep us updated...
     
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