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Author Topic: Maximum Antenna Height?  (Read 5745 times)
W5SRT
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Posts: 416




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« on: October 08, 2015, 09:11:11 AM »

I'm installing a Tarheel 100A in the bed of my pick-up, and it's OK to shorten the whip and still cover the bands I want (20-40-80 Meters). 

I'm thinking that the antenna whip should not go higher than 13'-6" above the pavement so that the truck can be driven under bridges without striking them.

Does that sound correct, or is there a better maximum height to use?

Thanks for your thoughts.

73

- Dan
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K0BG
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« Reply #1 on: October 08, 2015, 10:55:38 AM »

Instead of worrying about it, use a properly-mounted cap hat. Properly mounted‚at the very top of the antenna—and you increase efficiency, broaden out the bandwidth a bit, and reduce height all at once!
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W5SRT
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Posts: 416




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« Reply #2 on: October 09, 2015, 07:14:48 AM »

Don't really want a tree-branch catcher at the top of the whip, LOL.  Anyway, it looks like 13'-6" is a good maximum traveling height, so I was going to tune the Tarheel to 40meters and trim the whip at that setting, and then be a little more careful running 80 meters.

73

- Dan
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KG5ELA
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Posts: 26




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« Reply #3 on: October 09, 2015, 07:42:58 AM »

Do you go thru drive-thrus? Some of those clearances are only about 9'-0".
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Tim
N4NLQ
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« Reply #4 on: October 09, 2015, 08:02:12 AM »

You are correct in that maximum height for a non-permit tractor semi-trailer rig is 13 feet, six inches (13'6") but I have seen many highway and city clearances less than that by a significant amount.

Good thought about the drive through lanes, too.  I'd try to keep it down to 10 or 11 feet at most.

Allan _ N4NLQ
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W5SRT
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Posts: 416




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« Reply #5 on: October 09, 2015, 10:31:16 AM »

Do you go thru drive-thrus? Some of those clearances are only about 9'-0".

I know, gonna have to be careful when driving the truck.  But, with high blood pressure, my drive-through days are pretty much over.  Probably for the best anyway. LOL.
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W5SRT
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Posts: 416




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« Reply #6 on: October 09, 2015, 10:35:03 AM »

You are correct in that maximum height for a non-permit tractor semi-trailer rig is 13 feet, six inches (13'6") but I have seen many highway and city clearances less than that by a significant amount.

Good thought about the drive through lanes, too.  I'd try to keep it down to 10 or 11 feet at most.

Allan _ N4NLQ

I'll give that some thought.  Some of the state bridges aren't all that tall.  Seems like a little dragging of the whip at slower speeds wouldn't do much damage, dunno for sure.  Probably best to start at 13 foot and then trim further as necessary for clearance.

Thanks to everybody.

73

- Dan
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N3IDG
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Posts: 118




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« Reply #7 on: October 09, 2015, 04:26:46 PM »

13'6" is max legal height for non permit vehichles.  I have hun an antenna that was 14'2" at the tip and did fine with it on the work truck I did hit a few bridges with the tip but no damage at highway speeds. This was a 5/8 on 2 meters with very strong mount. Some states mark bridges a foot lower than actual some with the actual . Just find out how your state marks them.
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WN2C
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Posts: 550




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« Reply #8 on: October 09, 2015, 05:08:54 PM »

Instead of worrying about it, use a properly-mounted cap hat. Properly mounted‚at the very top of the antenna—and you increase efficiency, broaden out the bandwidth a bit, and reduce height all at once!

Alan has given you the best advise on here yet, but you worry more about catching branches. Have you looked at K0bg.com ?
Alan has a lot of very good info there for mobile operations.  And best of all it is free.

Rick  WN2C
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W5SRT
Member

Posts: 416




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« Reply #9 on: October 10, 2015, 05:56:23 AM »

Instead of worrying about it, use a properly-mounted cap hat. Properly mounted‚at the very top of the antenna—and you increase efficiency, broaden out the bandwidth a bit, and reduce height all at once!

Alan has given you the best advise on here yet, but you worry more about catching branches. Have you looked at K0bg.com ?
Alan has a lot of very good info there for mobile operations.  And best of all it is free.

Rick  WN2C

Thanks, I read K0BG's entire site and used it as guidance in my equipment selection and installation process.  I also talked with Tarheel Antenna about it too, they were very helpful as well.

73

- Dan
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WN2C
Member

Posts: 550




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« Reply #10 on: October 10, 2015, 05:50:30 PM »

Dan, I run (and have for the past 3 years) an Icom 7000 with a Scorpion screwdriver antenna, It works very well with a 6 foot whip. I make contacts all over and on every band 75 to even 6 meters. (using a setting that is close / works on 17 meters)  If you are looking for efficiency then you want a good antenna like the Scorpion with a larger diameter coil. A cap hat will increase efficiency and allow you shorten the overall height of the antenna but the Tarheel may not support the cap hat. You don't need an antenna whip 10 feet high to make contacts on 40 or even 75 meters, a 5 or 6 foot whip is all you need.

Rick  WN2C
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W5SRT
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Posts: 416




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« Reply #11 on: October 10, 2015, 06:18:43 PM »

I extended the Tarheel all the way, measured the height from pavement and trimmed it to just a hair under 13'-6"  That took 12" off the whip, and now the retracted height is just a touch over 12'.  That should be good.  BTW, this antennas has the Tarheel cap hat on it.

Checking the SWR and impedance mid-band on 15 through 80 meters.  I'm getting 2:1 on most, with 1.6:1 on 17 meter and only 2.8:1 on 40 meter (after twiddling with the matching coil).  The impedances are all less that |50| ohms.  The TS-480 tuner corrects from the radio perspective.  Tarheel claims 1.5:1 on all bands, and says grounding issues would drive it higher.  But, I feel like the grounding is very solid.  The receive noise floor is kind of high, but, the bands were pretty bad today.  There is some ignition whine, but the 480 noise blanker clears it up fairly well.  The 480 has the 1.8kHz crystal IF filter installed, which is a pretty big improvement (W0BG's recommendation followed on that).

Anyway, thanks for all the replies.  I'll be in touch with Tarheel on Monday to discuss ways to lower the SWR a bit.

73

- Dan

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K0BG
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« Reply #12 on: October 11, 2015, 06:16:52 AM »

The cap hat is great, but how is it mounted? If it isn't at the very top of the antenna, it is mounted incorrectly!
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K9PHT
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Posts: 79




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« Reply #13 on: October 14, 2015, 03:51:09 PM »

Back in the day we would mount the vertical at an angle on the front of the vehicle  and then pull the top of it down to a tie point.  You could get a lot of antenna is this type of install...

This is what the military did with their HF antennas on all of their vehicles...

Roy Ken
k9pht - King George VA
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K0BG
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« Reply #14 on: October 14, 2015, 04:01:50 PM »

at the military might indeed done it. But, does that make it the most efficient? Not by a long shot! While physical length is perhaps important. the real issue is ELECTRICAL length.
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