Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: HF, 2-meter, CB, on pickup-antenna mount question  (Read 4639 times)
KF5DEY
Member

Posts: 8




Ignore
« on: July 04, 2010, 09:21:06 AM »

Right now I have a dual band 2m/70cm antenna on one end of a headache rack, and a CB on the other end.   I am working on getting my General and would like to work HF using a screwdriver type antenna (High Sierra Sidekick, perhaps)


I assumed this question would be answered many times, but I have had difficulty finding it...

How far apart do the antennas need to be?

The only thing I have read is 1/4 wave spacing...well with HF, that 1/4 is still bigger than my truck...

I have had no problem between the 2meter and the CB, they are about 5 feet apart...actually I have no problems at all right now...pre HF.

I would like to mount the screwdriver on the headache rack between the two other antenna (so about 2.5' each way).

I don't have stake pockets.  I have a roll up aluminum cover that is locked closed most of the time, it covers the bed and the stake pockets. 

I do have a Thule rack that straddles the bed with two large bars, that I mount my bikes on.  It is possible that I could create a mount that would use that rack to mount the screwdriver in the center of the bed, above the bed cover...running the coax/bonding would be a pain from that location.

Obviously I can't get the optimal setup...but how about mounting the screwdriver on the headache rack between the other antennas vs floating above the bedcover?
Logged
K0BG
Member

Posts: 9845


WWW

Ignore
« Reply #1 on: July 04, 2010, 02:16:51 PM »

First off, a headache rack is a poor place for any HF mobile antenna, and especially so with an abbreviated one. You might want to spend some time on my web site.
Logged

KF5DEY
Member

Posts: 8




Ignore
« Reply #2 on: July 04, 2010, 03:08:34 PM »

I have.  There is no mention of the interaction of antennas...but it can't be any worse than mounting along the side of the cab off the frame can it?

BTW, by saying I would mount it on the headache rack...that is sort of misleading.

I would be mounting it actually behind the headache rack...but using that support... not perched above the headache rack.

I see a lot mounted on the frame about 2 feet off the ground, and along the side of the cab...some mounted in the first stake pocket behind the cab on the side.  Some mounted in the floor of the box.

Your ridgeline has it mounted on the floor of the box at the back.

It seems that with some of those mounts, the cab is blocking the RF...or is that not a factor in HF?
« Last Edit: July 04, 2010, 03:50:50 PM by Mike Barnett » Logged
KI4SDY
Member

Posts: 1452




Ignore
« Reply #3 on: July 04, 2010, 04:06:38 PM »

An old commercial radio installer told me long ago that you need at least four feet in between antennas to prevent noticeable interaction. I have used that rule of thumb for over 40 years with no problems. Wink
Logged
WX7G
Member

Posts: 5920




Ignore
« Reply #4 on: July 04, 2010, 05:16:03 PM »

What choice do you have but to mount them on the rack. As far apart as you can. Try it out. If one radio bothers the other it's time for a filter.
Logged
K0BG
Member

Posts: 9845


WWW

Ignore
« Reply #5 on: July 05, 2010, 06:25:20 AM »

There is so much misinformation on the web, it's difficult to separate the good from the bad.

The rules of thumb about spacing antennas doesn't take into account which frequencies we're taking about; what the front end capabilities the individual transceivers have; and perhaps about a dozen more. As Dave mentioned, place them where you can, or in some cases, where you have to. All this said, there are some important things to remember.

Almost any antenna will work, even those with efficiencies as low as 1%. That's about what twice the ERP of a hamstick on 80 meters. If you mounted it better, you might get 1%. Not much of a change to be sure, but that 3 dB increase might make the difference in making a contact or not. It just depends on how important that contact may be (which is a whole new topic).

VHF antennas aren't any different. Proper mounting might make the same 3 dB difference, but that fact doesn't necessarily mean you'll double your "talk" distance, especially on FM.

There is a hidden factor in both, and that is RFI. When the ground plane losses get high enough, you start having problems with common mode current. While you can choke off most of the common mode, if it can get out, it can get in too. It's difficult to imagine, but in most HF mobile installations, a good portion of the received noise is actually pickup by the coax, rather than the antenna. It isn't any different on VHF FM, except you can't hear the difference, but it nonetheless effects the SINAD presented to the transceiver; a factor which can be easily measured with a service monitor.

As for my Ridgeline install. The bottom of the truck is mostly plastic, as is the bed of the vehicle. This requires mounting the antenna higher up on the vehicle. Since I garage my truck, it needed to be mounted so the antenna would clear the door opening. Hence, the corner of the bed. This location does distort the pattern some, as any in-bed location will. Compared to the previous vehicle (Acura 3.2 CL), where the antenna was mounted high on the quarter panel, actual field strength measurements are essentially identical. Only their pattern is different. By the way, it is mounted directly above the main crossover which supports the rear suspension.
Logged

AA4PB
Member

Posts: 12697




Ignore
« Reply #6 on: July 05, 2010, 06:40:44 AM »

An old commercial radio installer told me long ago that you need at least four feet in between antennas to prevent noticeable interaction. I have used that rule of thumb for over 40 years with no problems. Wink

There is no fixed distance that applies equally to all frequencies. The rule of thumb is 1/4 wavelength. Obviously you can't meet that rule with an HF antenna so you use the greatest spacing that is practical and live with the results. Any HF antenna below 10M that you can drive down the road with is full of compromises anyway.

That installer's rule of thumb probably worked fine for him because he probably was installing VHF and UHF antennas.
Logged
VE7DQ
Member

Posts: 175




Ignore
« Reply #7 on: July 06, 2010, 12:50:58 PM »

K0BG de VE7DQ:  Alan, I'm confused.  You've always stated that it's the mass under the antenna that's important; wouldn't a headache rack on a pickup be the most efficient spot for an antenna (assuming the cab, box and rack are properly bonded)?  If not, why not?  What's the disadvantage (other than height)?

TIA and 73

Tom
Logged
K0BG
Member

Posts: 9845


WWW

Ignore
« Reply #8 on: July 06, 2010, 02:27:43 PM »

Another way to look at this is, where is the coax shield connection, with respect to the largest metal mass?

You can measure the input impedance of an antenna mounted on a headache rack easy enough. Move said antenna to the top of the bed rail, and the input impedance goes down in every case. Sometimes it's just a little, perhaps about one ohm, and sometimes it's a lot more. In one case I'm personally aware of, the drop was nearly 3 ohms. That amount of change is significant.

While you can't be absolutely assured the aforementioned change is due to a reduction in ground loss, it is, nonetheless, an assumption with merit.
Logged

KF5DEY
Member

Posts: 8




Ignore
« Reply #9 on: July 06, 2010, 04:09:42 PM »

I think my #1 concern was, (I can't think of the exact term) the output of the HF causing RFI through the Vhf/CB due to antenna placement...

I have been looking at hundreds of pictures of screwdriver type antennas on pickup trucks (and SUVs)...

I honestly don't see any rhyme or reason to their location other than convenience.

So worst case scenario, if I mounted the HF about 2.5 feet from the VHF, and about 5 feet from the CB, would I expect RFI due to proximity of the antenna?

Best case scenario, If I instead mounted the HF on the rear corner of the bumper, or perhaps on top of the bedrail at the rear corner, would I expect better numbers in the HF range there than up near the cab?

...and yes there will be a lot of bonding involved...especially with that 20' exhaust pipe.

BTW I get better VHF performance with the antenna on top of the headache rack vs on the cab...and RFI disappeared...I am assuming the headache rack on the truck bed is bonded...at least better than the cab.  I have work to do.
Logged
K0BG
Member

Posts: 9845


WWW

Ignore
« Reply #10 on: July 07, 2010, 05:44:30 AM »

My HF antenna is less than 24 inches away from the Navi, and the AM/FM antenna. I have no interference. Again, the main problem is not closeness of the antennas which cause the most problems. Rather it is common mode current flowing on the outside of the coax of any of the antennas. You can choke off the CM, but the best way is proper mounting which minimizes the problem in the first place.

http://www.k0bg.com/common.html
Logged

KF5DEY
Member

Posts: 8




Ignore
« Reply #11 on: July 07, 2010, 08:51:42 PM »

Thanks, the info has eased some of my concerns!
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!