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Author Topic: RV roof ladder as an HF antenna!  (Read 5237 times)
K3RBP
Member

Posts: 15




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« on: December 10, 2001, 11:25:39 AM »

I want to install an HF rig in my motorhome, so I was considering the various antenna choices... screwdriver, Iron Horse or Hustler, Ham Stik, etc. As I was looking where I would mount this antenna, a strange idea came to me.

I have a 32' fiberglass-shell motorhome with a permanent-mount anodized aluminum roof ladder on the back. The bottom of the ladder is about 4 feet above ground, and has 88" long rails that have a 90 degree sweep bend at the bottom, which attaches to the rear wall of the motorhome above the bumper. (this bend adds about 8 inches to the previous length). There is another 90 degree bend at the top which then extends 16 inches forward, and finally another 90 which adds 8 more inches, and bolts through the top of the fiberglass roof.

The ladder is insulated from the chassis and metal frame, with no resistance found on even the highest (20 meg-ohm) scale on a digital meter. So I have been thinking about connecting feedline to the bottom of the ladder, grounding the shield to a frame rail nearby, and then installing a marine type lay-down (ratchet) mount on the roof, near the top of the ladder. I would mount a Hustler or other resonator and SS whip, of a length yet to be determined, on the ratchet mount, then connect the bottom of the resonator to the top of the ladder.

Is this feasible? I want to work 20 meters mainly, but basically will go for whatever works best. Would a Hustler 15 or 20 meter resonator work with this arrangement? BTW, the rig that I'm going to use (for now) is a Kenwood TS130-SE. I also have a never used MFJ-940C antenna tuner, which I could use, or I may buy an automatic tuner. Anybody out care to share their thoughts on this?
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WB2WIK
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Posts: 20595




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« Reply #1 on: December 10, 2001, 04:55:15 PM »

I'd say "try it."  I suspect the Hustler resonators may all have too much inductance because they are intended to be used with a much shorter mast than the 88"+ than you have, but because of all the variables in your system (length, bends, attachments, possible coupling through the fibreglas to conductive components inside the motorhome, etc, etc) it would really be impossible to model this; the real test would be to try it.

Which, in your case, wouldn't be hard nor expensive.  You'd need a 3/8-24" stud mount attached (mechanically and electrically, both) to the top of your ladder and a coaxial feedpoint attachment at the bottom of the ladder; a good, low-impedance ground strap; and a Hustler resonator/whip, which only cost about $20.

I'd strongly recommend, though: Forget about the tuner, and focus on resonating the antenna, whatever that takes.  Loaded systems such as the Hustler mobile resonators provide are very hi-Q and need to be tuned for resonance, without a tuner, to provide any degree of efficiency or performance.  I use Hustler mobile resonators on one of their standard foldover masts (which are only about 54" tall), and while it is true I can set the whip length "anywhere," and then use an antenna tuner to dip the system for a perfect VSWR, when doing so, I can't hear or work anyone.  The whip length adjustment is extremely critical (1/8" makes quite a difference), and until properly set, the antenna is a dummy load.  When it is properly set, a tuner is not needed.

73 de Steve WB2WIK/6
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K3RBP
Member

Posts: 15




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« Reply #2 on: December 10, 2001, 05:59:59 PM »

Thanks for those ideas Steve. Yes, I'd rather have the antenna resonant, as the radiation efficiency is certainly much higher than a non-resonant load with a tuner used to produce a low SWR. I guess this might be a good application for an analyzer like the MFJ259B. I don't have one; I have an old MFJ antenna noise bridge and a freq counter, which would probably help here, although the 259B would tell me more of what I need to know to adjust the variables, and bring it into resonance. It will be an interesting experiment.

73, Bill
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KC5JK
Member

Posts: 98


WWW

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« Reply #3 on: December 29, 2001, 07:32:28 PM »

I would forget about adding mobile resonators.  First, find out (with the analyzer) where the ladder resonates as-is.  You might just get lucky and find that you've already got a complete antenna up there.  More likely, you will have to add some loading (either a variable inductor or tapped coil) to get it to dip on one or more desired bands.  This could be in series with the feedline at the fed end of the ladder, or at the other end (as in "top hat").
Please post your findings, if something works out, and specify the RV and maybe ladder make and model, for other travellers who might have the same road set-up that you have.
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WA2MZF
Member

Posts: 22




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« Reply #4 on: March 05, 2002, 09:50:53 AM »

I am a full time rver and have used all kinds of ants.. so much for that statement.
What I want to mention is that in our 5th wheel which is also fiberglass. I have
tried all kinds of ants with no success on the ladder, The main point being is the equipment is
located in the rear about ten feet or so from the ladder and ants.. YOU will get rf feedback this
close ! Since I run all ants from the front which is about 30 plus feet away from the
radios, I do not have any problems at all.  I have run the screwdriver, hust res etc.
My ant of choice now is the hamsticks for many reasons. I also run an elec
power company disconnect pole on the front which goes up about 30 feet thus
i use rotabable hamsticks. reports sometimes are that people think i am running
a beam. Back to the point, I have found that most anything will load up ! I have used
ants that in theory should not have worked but did just fine.. As someone else
pointed out "TRY it" There are some links, pictures of what I use on my home
page if you wish to look. http://www.qsl.net/wa2mzf  Also we do hf pactor
for our email on the road. This system has served us well for over four years now
when we do not have a phone ! Again info on that on our home page.

An additional side note. I also pulled off the rubber cap so to speak which seals the roof
to the side, all rvs have this. And installed a 34 foot wire on each side of the coach
thinking of using this for an antenna. The problem here again is too much rf near the
rig. You just have to get the ants as far from the rig as you can when living in
a fiberglass house ! hi.. 73  jim  wa2mzf
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VE7FED
Member

Posts: 1




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« Reply #5 on: April 23, 2002, 09:30:53 PM »

That was great reading and covered most of my first thoughts, but but hasn't helped me.

I have a 1997 30' Tripple E Commander, a Canadian made well insulated Class A gas efi engined coach with steel frame, and the ladder and luggage rack read zero ohms to battery negative! As the roof is 11' above the road any roof mounted antennas would have to be pretty short! I thought about an end fed horizontal wire, but again, the (grounded)steel frame would be very close. A side, front or rear mounted mobile whip would also probably  be adversly affected by the steel framework. I have an Icom IC735 and AH2 tuner I am hoping to use. Any one out there have any thoughts?Huh I have a second call sign to allow me to have ham plates on the RV as well on my car: call is VA7RV and I can't let that go to waste!!!!!

HELP!

thanks & 73

Art
VE7FED (and VA7RV)
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