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Mid roof Volvo sleeper

Discussion in 'CB Antennas' started by skip searcher, Apr 16, 2015.

  1. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    Interesting concept, post a pic if you have some.


     

  2. The DB

    The DB Sr. Member

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    Are you sure I need to do said reading here?

    I am well aware of the effects of loading an antenna. Electrically a loading coil is a current device, and by extension, will be more effective where there is more current. This is well known and is written about in multiple respected publications, including but not limited to the ARRL Antenna Book series, recent versions of which give you direct formulas for creating a coil for the overall length of antenna you want, including where along the length of the antenna you plan on putting the coil. If moving the coil on the antenna has no effect on how much the antenna is shortened by it, why would such formulas that include where the loading coil is on the antenna be necessary?

    These same changes can also be shown in models, both the change in efficiency, and thus its effect on performance, and the change in reactance and impedance, and the only change modeled was simply moving the loading coil on the antenna...


    The DB
     
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  3. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    I said this because you are saying a here a coil at the base is more effective shortening the antenna and that would be a base loading antenna. That's usually the case,

    what we are talking about here is Predator and other center loading antennas and lengthening the lower shafts some to maybe shorten the antenna length while keeping it a center loading type antenna. So it kind of sounds like you would recommend converting the antenna to the least efficient antenna type over efficiency for a lower antenna length. A base loaded antenna has a small current over the entire antenna length. The least effective compared to center or top loading and can be described as a leaking dummy load for an analogy.

    The other hand you acknowledge that moving the coil up as in a center load is more efficient.

    That's correct on efficiency, A center loaded antenna has a high current flowing in the bottom portion of the antenna and a lower current in the top section. This is commonly known as the best compromise next to top loading since most mobile antennas aren't truly top loaded except when using a cap hat like I run on my mobile HF antenna.

    Does moving the coil up shorten the antenna up? not hardly in my past experiences doing it with Predator, Hustler, and now my Sirio antenna. They have all generally re-tuned the same but required cutting the whip. I have lengthened the lower shaft as much as 3 ft. on the Sirio antenna and didn't see much of a change since the stock whip is 71 inches. Predators antennas I have used with 9 and 27 inch shafts and saw not much difference in overall length after tuning.
    This is a reflection on what each antennas loading coil can match up to and this is what I was pointing out to the OP when he was considering a smaller shaft antenna instead of his Wilson extended shaft (22 inch) antenna that the lower shaft size on the same antenna won't make much if any difference lengthwise after tuning.

    To significantly shorten a center loading antenna up and match it up to a 50 ohm load would be to use more inductance in the loading coil which is counter-effective since it will decrease efficiency, or better yet, make a homebrew cap hat on a short whip. Not many people in the CB world would go to such lengths.
     
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  4. The DB

    The DB Sr. Member

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    I don't think that we are really that different in what we are saying that much. I did mention above...

    Due to the current distribution, changing the shaft lengths, even using a three foot shaft (over a much shorter shaft, say 6 inch), won't have a serious affect on the effective overall length of the antenna, or the efficiency of the coil on an antenna of this overall length at this frequency. There will be some, but not so much that you would notice...

    If you want to make enough of a difference that you would notice for either you need to put the coil much further up the antenna, to the point that in most cases it would be impractical. That two or three foot at CB frequencies, while it has other benefits, just isn't going to affect the efficiency or the length of the antenna enough to notice the difference in the real world, even compared to a base load.

    I'm pretty sure that center load they refer to in the ARRL Antenna Book as a good compromise if you can't top load the antenna is specifically referring to an equal length of antenna on either side of the coil. There are several reasons why that would be the case, although I will have to look it up to confirm if they specifically mention that.

    Also, if you add a coil to match to a 50 ohm feedline, this would be counter productive unless you have lots of losses in the system as a coil lowers the R variable at resonance, not raises it. The losses problem is the case with pretty much all vehicles today as very few if any vehicles are present a remotely adequate "other half" of a mobile antenna without modification. That cap hat you mentioned, however, would raise R at resonance, and is more efficient than a coil for a given amount of shortening (in a vast majority of cases). Some of the most effective shortened antennas actually use a combination of both a coil and a cap hat...

    Edit---
    I should have simply said that as both the efficiency and antenna length effects of where the loading coil is along the antenna length changes are both related to the amount of current available, the changes in antenna length required for changing the position in the coil are a good indicator of how much more/less efficient the antenna now is.

    Edit 2---

    The DB
     
    #19 The DB, Apr 18, 2015
    Last edited: Apr 18, 2015
  5. rabbiporkchop

    rabbiporkchop Sr. Member

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    This is the only one I could find. it is very small and you need to look closely.[​IMG]
     
  6. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    I really can't see it. I would assume you mounted them to the side fairing grab handles.
     
  7. rabbiporkchop

    rabbiporkchop Sr. Member

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    QUOTE="fourstringburn, post: 518641, member: 2920"]I really can't see it. I would assume you mounted them to the side fairing grab handles.[/QUOTE]
    I made 2 L-brackets and I slipped them in between the upper half and the lower half of the fairings. they were grounded to the reinforcement bracket which connects to the sheet metal on the back of the cab and prevents the fairing from flexing. this only works on the full size Volvos and not the smaller trucks. no grab handles were involved
     
  8. skip searcher

    skip searcher Active Member

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    I can't see anything but I might look into one of those grab handle mounts
     
  9. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    It sounds like you did it right. Too bad you can't blow up that pic some because we can't see it.
     
  10. rabbiporkchop

    rabbiporkchop Sr. Member

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    Its really easy to duplicate. All you need are a couple of homemade angle brackets to attach in between the 2 layers of fairing and a matched pair of elements
     
  11. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    Ok, I get how you mounted it. That was what I wondered about.

    That is a clever alternative that will work. I looked on my truck and between the upper split fairngs, there is a gap, minimal it is but enough to put an angle brace bracket to mount a antenna. I will keep that in mind for future reference.
     
  12. rabbiporkchop

    rabbiporkchop Sr. Member

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    [​IMG]
    Here is a photoshop image with better clarity.
    E="fourstringburn, post: 518556, member: 2920"]Interesting concept, post a pic if you have some.[/QUOTE]
    Heres
     
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