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Author Topic: Mobile HF Antenna Installation Questions and Recommendations  (Read 6341 times)
K4SSS
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Posts: 5




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« on: November 03, 2011, 09:37:09 PM »

Greetings all.

I could use and would very much appreciate any thoughts or advice that any of you mobile HF ops experts out there have regarding a mobile HF antenna installation that I am in the design stages with right now.  In a nutshell, my present mobile rig is an Icom IC-7000, using a 2m/440 whip, and I hope to add an Icom AH-4 tuner and an Icom AH-2b antenna element, both of which I already own, and use this combo to work 7-54 mhz (75/80m would be a dream come true!).

I drive a 2008 Nissan Xterra, and the best place I have found to mount an HF antenna is on the left rear quarter panel, where a small, removable, 5"x8" indented plastic panel mounted on the plastic rear quarter panel (yep, plastic).  The panel is used as a step-well to help you access the roof rack, and it covers a well built metal step which is mounted to sheet metal body right behind the plastic panel.  My plan is to either remove the 4 large bolts that hold the step on and install a custom made mount in its place that would extend out sufficiently to mount an antenna, or to simply bolt or weld some 1/4" steel plate stock on top of the existing step.  In either case, given this design, the antenna mount and feed point would be approximately 30" above the ground, and the antenna would be about 6" minimum from the side of the automobile. Luckily, there is also enough room behind the very aft left interior panel to mount my AH-4 antenna tuner, and I  estimate that the RF output lead from the AH-4 tuner to the AH-2b antenna connection will be about 2-3' max.

At present, my IC-7000 is mounted under my drivers seat, and the remote head and its mounting bracket are fitted to plastic piece that I fabricated (gotta keep the xyl happy...) and mounted on the lower dash. For power, I used a 13' long, 8 gauge red cable connected to my positive battery terminal, fused it next to the terminal, and then ran it thru the firewall directly to the IC-7000 power input connector, were I also fused it.  As for the DC negative lead, it specifically says in the Nissan manual not to hook any aftermarket accessory negative power leads directly to the negative battery terminal due to possible problems related to the battery charging circuitry not charging the battery sufficiently.  Hence, my IC-7000's  DC negative power lead (8 gauge) is attached to the car (and fused) near the radio,  and the IC-7000's RF ground (1" braid) is attached to the car close to the radio too.

For those not familiar with the Icom AH-2b antenna element, it may help me if I explain its design before going any further.  The antenna itself is supplied in two separate pieces:  the upper 98.5" (2.5 meters) long stainless steel (SS) tapered whip (.237" or 6mm in diameter at its base);  and the lower, integrated, factory built base assembly into which the whip is inserted.  This base assembly is darn rugged, and measures 13" long total.  It is comprised of, from bottom-to-top, of a large round plastic 3-3/8" W x 2-1/2" H insulator with a threaded hole on the bottom to attach your coax center conductor;  a large, HD 4-1/8" H SS spring; and a rugged, 6-3/8 H SS shaft, with holes for two set screws.  It is this upper SS shaft in which the SS whip is inserted, via a hole drilled down the shaft center.  If you insert the whip into the shaft until it bottoms out (5-1/4"), the total length of the antenna will be 106-1/2" from the bottom feed point attachment to the end of the corona tip.  If you insert the SS whip into the SS shaft only 3-1/4", which is more than sufficient for the two set screws to engage the shaft securely, the overall antenna length will be 108.5".  All in all, the completed antenna is a pretty rugged set-up.

(Note:  Since I don't intend on using the supplied base bracket, tow hook bracket, or other components supplied with the AH-2b in this design, I won't be describing them here.  But, if you desire info regarding them, please contact me.)

OK, down to business.  Here are a few questions that I would like to ask all you mobile ops experts out there regarding this particular set-up.

1.  I have installed and helped install numerous mobile setups over the past 40+ years, but this is the very first time that I have not attached the main DC negative power lead from the radio directly to the negative battery terminal!  Have any of you done this before, and, if so, do you have any advice to offer me regarding this type wiring setup?  How about the attachment point of the IC-7000's main DC power negative lead being in near proximity to the IC-7000's RF ground attachment point --- anything to offer with regard to that set-up?  It just seems odd to this old geezer, hi!

2.  As I said, my antenna feed point (coax attachment to antenna) will be about 30" above the ground given my present design. From that point it is about 47" to my roofline, meaning that just the top 61" of the whip will be exposed above the roof line, in free space (108" antenna length minus 47" = 61").  Any advice on if you would recommend my either lowering or raising the feed point height given this set-up?  Conventional wisdom might say that I should lower it, but each installation is unique, and I'd appreciate your thoughts regarding what you might do if given this particular installation.

3.  I'm guessing that this question might generate more comments than the above questions, hi, but I'm wondering what your thoughts are regarding my mounting a screwdriver antenna (versus my using the AH-4 tuner/AH-2b antenna combo) in the very same location on my Xterra?   I know that this topic (whip versus screwdriver) has been addressed here before and in other radio forums, but I thought that it wouldn't hurt to ask it again, given my particular installation on my Nissan Xterra.

I'm certain that I'll have some other questions, but right now my biggest concern is getting the best efficiency possible out of my new mobile set-up.

Any comments or advice that you might also have regarding the use and best location for items such as chokes, etc., in this particular arrangement would also be appreciated.

Lastly, I'll add that I've spent the last couple months researching this subject on the internet and elsewhere.  What I discovered is that there is a wealth of really good info out being provided by some really intelligent and dedicated hams, and I'd like to take this opportunity to say thank you to them for all of their hard work.  They are a real assets to our community!

Thanks to all in advance for your help in this matter, and best regards.

Bob -- K4SSS
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VE3XKD
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Posts: 51




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« Reply #1 on: November 04, 2011, 02:50:13 AM »

Bob

One thought, I use the Icom 7000 on HF, it is mounted much like yours, radio under the seat and panel attached to a bolt on gooseneck mount. I routed the display to myPrius navigation display and it is much easier to see and operate this way. I don't know if your Nissan has a display but it is worth considering.

With the radio mounted under the seat, it can also be areal pain to access any wiring,  I found that I needed to tie wrap all wires in place to keep things neat and stable, I also labelled all wires so that I could see and troubleshoot everything without having to take the seat out.

Good luck on Your mobile. I am upgrading my antenna to incorporate a larger cap hat as I find I am having more fun mobile theses days than in he shack.

73

John
VE3XKD
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N8EMR
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Posts: 234




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« Reply #2 on: November 04, 2011, 03:37:14 AM »

The AH4 and the AH2b is not going to work well on 75m.... Just not long enough whip.  Your ok to 40m and the AH4 will tune 75m does not going to radiate well.  I personally would not get the AH4 for mobile. Go with an SGC tuner and possibly a longer whip.

Take a look at the AH4 specs you will see that it takes a 23ft whip for the Ah4 to properly tune 3.5mhz, but will tune 40m with the ah2b.

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K0BG
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« Reply #3 on: November 04, 2011, 06:14:13 AM »

I think you might have misinterpreted what Nissan suggested. The ground lead should be connected to the chassis ground point near the battery. Using the chassis could cause some ground loop problems, you'll swear are RFI issues. Nissan uses a really funny Battery monitoring system, hence their recommendation.

There is nothing wrong with the AH-4, but when coupled to the AH-2, you need to follow Icom's recommendations to the letter. Remember too, the output side will have rather high RF voltage imposed, so you have to be careful. The leads need to be short. That's inches, not feet! And, they can't be coax.

You'll also need to choke both the coax, and the control leads to the AH-4, or you're going to have problems with common mode. You might want to read this: http://www.k0bg.com/couplers.html
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WX7G
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« Reply #4 on: November 04, 2011, 08:16:02 AM »

Question:
2.  As I said, my antenna feed point (coax attachment to antenna) will be about 30" above the ground given my present design. From that point it is about 47" to my roofline, meaning that just the top 61" of the whip will be exposed above the roof line, in free space (108" antenna length minus 47" = 61").  Any advice on if you would recommend my either lowering or raising the feed point height given this set-up?  Conventional wisdom might say that I should lower it, but each installation is unique, and I'd appreciate your thoughts regarding what you might do if given this particular installation.

Answer:
For highest radiation efficiency the base of the antenna (the tuner) should be at the top of the vehicle. The "antenna" and vehicle form a vertical dipole. Mount the 108" whip on top of a 72" vehicle and you have a 180" antenna. Mount the 108" whip with 61" above the roof line and you have a 133" antenna. The radiation resistance will be roughly 1/2. Additionally, with the lower mounting there will be higher loading coil (turner) current and higher loading coil losses due to coupling between the portion of the whip that is parallel to the vehicle.

All of this can be modeled using NEC so that comparisons can be made between different mounting heights and whip lengths.
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K4SSS
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« Reply #5 on: November 04, 2011, 10:11:23 AM »

Hi Guys, thanks for your responses.

-- VE3XKD.  Hi John. Your being able to display your IC-7000 info on your nav display sounds awesome!  But, nope, unfortunately, I don't have that luxury, hi...  My under seat wiring hasn't proven to be a problem just yet, and, given my past mobile installations, has actually proven pretty reasonable, and the space under my Xterra's driver's seat is extremely well protected from either my feet or a rear passenger's feet contacting anything.  I plan on experimenting with a cap after my installation.  73's

-- N8EMR.  Hi Gary.  Yep, per the book, and if you do the math, the combo will only work 7-54mhz.  That said, while camping I can attach a long wire (> 23' total) to the whip and work 75/80m.  73's

--  K0BG.  Hi Al.  Do you ever sleep, hi?  Many thanks for your outstanding community support!  You are obviously correct about the AH-4 RF output being wire versus coax (my mis-speak).  IAW the AH-2b directions, however, Icom states that the tuner unit to antenna base wire should be "50 centimeters (about 19 inches) or longer," whereas you indicate that it should be just inches...  Now, granted, the AH-2b instructions make no mention of the AH-4, and only the AH-3 or AH-2a tuners, which may account for the difference.  What are your thoughts regarding this?

Also, I am curious as to what type wire (gauge, covered, bare, solid, stranded, etc) you would you recommend for the tuner RF output to AH-2b connection?  (note: the AH-4 is rated 120w max input)

I'm ready to take on any potential common mode on both the control and coax to the AH-4.

With regard to the ground lead:  The Nissan factory set-up has a ground lead running from the negative terminal directly to the engine block (about 1 foot), and not to any chassis connection.  About 5 inches down this lead there is some plastic contraption wrapped around the lead which almost looks inductive in nature, and it has several much smaller gauge wires running from it. Then, after that contraption, about halfway down the ground lead, it is stripped bare for about 1 inch, and a lug is attached.  This lug is then directly bolted to a small metal support. 

Given this anecdotal info (without a picture...), would you suggest that I run my IC-7000 negative DC power lead to this engine block connection, versus doing what I'm doing right now?  I haven't had any ground loop/rfi issues so far with just my 2m/440 set-up, but I certainly agree with your saying that moving the lead from where I have it now back as close to the battery as possible will help minimize any potential ground loop/rfi with my new, higher powered HF installation.

Again, thanks for your help Al, and 73's.
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K4SSS
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« Reply #6 on: November 04, 2011, 10:37:51 AM »

For WX7G. Hi Dave.  One of my very first mobile installations, back in 65/66 as I best recall, mounting a 6m Halo top, dead center on our families Ford Country Squire.  It worked liked a bandit, though it sure did get A LOT of very funny stares back then, hi!.  That, and the old 50' Vista tower!  Most certainly, top, dead center is probably the best location, but unfortunately, as you are well aware, mounting a 108" whip (or anything else for that matter) on the top of an SUV is only a disaster waiting to happen, and so, you've got to work with what you have on your particular automobile type: read -- compromise.   

I looked at mounting my AH-2b (or a screwdriver) on my trailer hitch, using one of the various tilt-over rigs that are available, but since I make so much use of my Xterra's rear storage area, I ruled it out for that one reason alone.  The AH-2b does come with a tow hook bracket which allows you to connect it directly to most any vehicle tow hook or available frame hole, but I ruled that out due to the low feedpoint.  Hence, I decided to go with the rear quarter panel option, which I've used with good success in many past installs.

The fun part about all of this is that I love experimenting, which is a personal attribute we amateurs must have if we're gonna enjoy our hobby to any degree.

73's
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VE7EPP
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« Reply #7 on: August 14, 2012, 12:43:33 AM »

Bob, sorry for the off topic but I wanted to contact you... was it you that restored a Collins 30J in 2002?  I have one here with the original manual, 30J-18 that I am going to restore. david@eppert.ca ve7epp
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N6AJR
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« Reply #8 on: August 27, 2012, 10:16:26 AM »

I normally do my mobile installs with a direct positive  cable and negative cable direct from the battery, with a fuse on or close to the battery,  then I also ground the negative post to the fender at the battery, and the neg side to the body at the radio,I use a self tapping sheet metal screw and a piece of flat braid from the ground on the radio to the chassis right at the radio.  this gives the neg side  botha a power return and also prevents any difference in potential from the radio to the battery.  it works well for me.  I also ground the  shield side of the antenna to the chassis with a short piece of braid so the cold side of the antenna is in ground with the car and the radio.
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KC2KMJ
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« Reply #9 on: August 28, 2012, 09:06:00 AM »

I wouldn't mount the radio under a seat due to possible heat issues. If you've got nowhere else to mount it, then that's another thing, but keep it in mind while Tx. I agree about running both a positive and negative cable to the battery, both fused as close to the battery as possible. I've never had a problem with ground loops or poor radio performance doing it this way.
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