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] 2 Next   Go Down
  Print  
Author Topic: VHF Base Antenna Choice  (Read 1473 times)
K2DWB
Member

Posts: 15




Ignore
« on: January 23, 2010, 03:25:04 PM »

I'm looking to improve my 2m simplex range in a particular direction - but significant terrain (a hill) is between my antenna mount and the other fixed station I wish to communicate with.  Right now, I have a J-Pole antenna mounted about 18 feet off the ground - and I'm just barely able to copy the other station on simplex. Terrain is clearly the issue - if I drive just 1/10 mile away, I can easily communicate with the station in question via my mobile radio.

I understand that J-Poles largely radiate at a fairly low angle.....and I'm curious if changing to a 2 x 5/8 base antenna would likely make much of a difference or if a 7/8 wave antenna would be better.  Thus far, I've not been able to discern the radiation patterns for these types antennas is (although I know that a single 5/8 wave antenna has a relatively low angle of radiation).

Anyone have suggestions?  I hope to not raise the base of the antenna any further (that would be an obvious choice) so as to preserve the appearance of my backyard.
Logged
N4CR
Member

Posts: 1672




Ignore
« Reply #1 on: January 23, 2010, 04:16:32 PM »

I'm looking to improve my 2m simplex range in a particular direction - but significant terrain (a hill) is between my antenna mount and the other fixed station I wish to communicate with.  Right now, I have a J-Pole antenna mounted about 18 feet off the ground - and I'm just barely able to copy the other station on simplex. Terrain is clearly the issue - if I drive just 1/10 mile away, I can easily communicate with the station in question via my mobile radio.

I understand that J-Poles largely radiate at a fairly low angle.....and I'm curious if changing to a 2 x 5/8 base antenna would likely make much of a difference or if a 7/8 wave antenna would be better.  Thus far, I've not been able to discern the radiation patterns for these types antennas is (although I know that a single 5/8 wave antenna has a relatively low angle of radiation).

Anyone have suggestions?  I hope to not raise the base of the antenna any further (that would be an obvious choice) so as to preserve the appearance of my backyard.

If you have a physical obstacle, antenna gain won't fix the problem. You need to move your antenna left, right or up. You might be surprised with a small move. I have seen signal jump when an antenna was moved just 2 feet. So first, see if there's a better place for your antenna near where it is currently located. If not, about the only choice is to go up.
Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W5FYI
Member

Posts: 1046




Ignore
« Reply #2 on: January 23, 2010, 09:00:35 PM »

Can you mount a ½-wave vertical at the top of the hill to act as a passive repeater? Something like 38 inches of straight wire or rod suspended vertically from a tree branch (for 146.5 MHz)? It just might be all you need. Otherwise, a ½-wavelength center-fed vertical dipole mounted high up will have a low take-off angle, and will keep your radiation pattern close to the horizon.
Logged
WB6BYU
Member

Posts: 13333




Ignore
« Reply #3 on: January 23, 2010, 11:28:22 PM »

Any well-designed omnidirectional vertical will have a reasonably low angle
of radiation.  The gain increases with length, which also flattens the pattern
so more and more of the radiation is right at the horizon rather than at 20
or 30 degrees above it (that is how you get omnidirectional gain.)

When the other station talks, does your S-meter vary with the voice?  If so
you probably have multi-path (the RF is reaching your antenna via two
different paths - perhaps bounding off of two different hills.)  If so, moving
the antenna a foot or two may make a big difference.

Using a taller antenna will give you a strong signal, but not by all that
much.  Raising the antenna makes the biggest difference, but if you
have to keep the overall height constant then using some sort of beam
will allow you to focus more signal in the desired direction (and helps on
receive, too.)  You can get as much gain using a simple 3-element yagi as
you can with a 20' tall omnidirectional antenna on 2m, and larger combinations
such as a pair of 6 to 10 element yagis side-by-side (with the proper spacing)
can deliver considerably more.
Logged
NE3R
Member

Posts: 11




Ignore
« Reply #4 on: January 24, 2010, 06:51:15 AM »

Hight is your best solution here.  If you went with a directional antenna, you'll improve your signal in one direction, but it will be reduced in others.  So you would want to put it on a rotator.  If you can't get your antenna any higher, an amp and preamp might do the trick.

73 de Joseph M. Durnal NE3R
Logged
K2DWB
Member

Posts: 15




Ignore
« Reply #5 on: January 24, 2010, 05:24:36 PM »

Great!  Thanks very much to all who responded.  I appreciate your input.
Logged
K6CMJ
Member

Posts: 32




Ignore
« Reply #6 on: January 24, 2010, 07:16:47 PM »

Why don't you try a stacked J-pole. They are very easy to construct and each one you stack adds gain. Mine has 4 stacked J-poles and is 10 feet tall. I'm able to communicate 100 miles away over hilly terrain with 10 watts. For the design, google stacked jpole for 2m.
Logged
KQ6Q
Member

Posts: 988




Ignore
« Reply #7 on: January 24, 2010, 07:47:18 PM »

If it's just one station you want to improve the connection with, get or make a beam and try getting a signal reflection on something that isn't in the direct path - water tower, grain elevator, tall building - anything that both of you can point a beam at. It could even be further away from you than the station you want to talk to, but if you both point your antennas at it, it can work. Focus your signal with beams - you can even homebrew a 2 or 3 element beam for 2m quite cheaply. See the ARRL Handbook.

Fred, KQ6Q
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #8 on: January 24, 2010, 07:48:42 PM »

"A (stacked or regular) J-pole has good low angle gain" is a statement that's very dependent on how much current it's driving on the outside of the coax feedline, which is in turn very dependent on the exact length of the coax, how it terminates at the radios or grounding system or whatever, and its position with respect to all objects it passes by.  It's very easy to drive significant common mode current on the coax, even if you put a choke at the J-pole feedpoint.  This means there's a pretty healthy dose of luck involved with J-pole performance.

The first thing I would do is try to replace the J-pole with a simple ground plane with radials, with some ferrite cores snapped on the coax below the feedpoint, just as a check that you're not wasting power in a bad-luck installation.  

If that doesn't improve anything, then you know your J-pole was doing OK as far as the 1/2 wave antenna bit being the most important radiating part.  Then you can start thinking about directivity, and I'd probably  add azimuthal directivity in the form of a beam.  

I know you've concluded that terrain is the fundamental  issue, but I can envision a situation in which neither your mobile (properly functioning antenna, but very low to the ground) or your J-pole (not radiating where you want it) can contact your distant station when you're at home, but your mobile antenna works great when you go to a different location.

It's this sort of marginal situation in which otherwise un-noticeable performance problems really start to matter, and the luck-of-the-draw element of J-pole performance is worth looking into.

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #9 on: January 24, 2010, 07:56:00 PM »

If it's just one station you want to improve the connection with, get or make a beam and try getting a signal reflection on something that isn't in the direct path - water tower, grain elevator, tall building - anything that both of you can point a beam at.

Airplanes work GREAT, but the darn things won't stay put.

Despite that I'm pretty sure I've made several quick contest QSOs on 2m and 432 CW that I just wouldn't have made without airplanes. 

Here's a spectrogram of doppler shifted airplane scatter surrounding the WA1ZMS beacon on 144.185 here:

http://n3ox.net/files/WA1ZMS_0042Z_082309.jpg

You just need to hang a big aluminized mylar balloon a hundred feet above your hill :-)

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #10 on: January 25, 2010, 02:54:42 AM »

Airplanes work GREAT, but the darn things won't stay put.

Despite that I'm pretty sure I've made several quick contest QSOs on 2m and 432 CW that I just wouldn't have made without airplanes. 

Here's a spectrogram of doppler shifted airplane scatter surrounding the WA1ZMS beacon on 144.185 here:

http://n3ox.net/files/WA1ZMS_0042Z_082309.jpg

You just need to hang a big aluminized mylar balloon a hundred feet above your hill :-)

73
Dan

"Big" would be the key word, and to be effective you have to use a beam antenna pointed at the passive reflector or "passive repeater". The amount of coupling between a dipole 500 wavelengths away and an omni-antenna is virtually immeasureable.

Very large area objects work well because they can be broadly illuminated, that's why a hill will "knife edge" a signal at VHF and higher.

If it was my system I would either get a small yagi antenna, try to raise or lower the antenna I already had, or buy a good used or new collinear antenna. It's possible to build a collinear, but quite frankly most designs published for Hams are not very good. Many don't have gain at all. Say we stack two good 1/2 wave antennas perfectly end-to-end. An antenna will always have less than 2 db gain if we do everything perfectly! If we stack two 5/8ths waves, we can get anything from negative gain to perhaps 2.5 dB gain maximum.

There is a stacking gain graph here:

http://www.w8ji.com/stacking_broadside_collinear.htm


This is why stacking antennas doesn't really do much at all unless the antenna is huge. If you can barely hear the other fellow most likely you need about 5-6 dB gain or more. You can't afford to do anything wrong, because unless the antenna you have now is a poor antenna that much gain will be very difficult to obtain. A collinear that provides 6 dBd gain would be about 25 feet long on two meters, that's about the same gain as a small three element yagi that you could hold with one hand.


Logged
WA3SKN
Member

Posts: 5494




Ignore
« Reply #11 on: January 25, 2010, 04:50:27 AM »

OK, simplex signals are at noise level, and you will want about 5-6 dB of gain for comfortable listening.  A small 3-5 element beam will do this for you.  They are fairly cheap and easy to come by, and are even easily buildable.  You will still want an omnidirectional antenna, though, so an extra coax run or switch would probably be desired.
Good luck with the project!
73s.

-Mike.
Logged
K0BG
Member

Posts: 9878


WWW

Ignore
« Reply #12 on: January 25, 2010, 06:31:01 AM »

This is a bit of aside, but important nonetheless. I'll protect the guilty!

Here are these three wonderful guys, living in a small mountain town, who really wanted to talk into the Denver, area repeaters by installing a passive repeater atop a nearby mountain. The discussion went on for months, as to the antennas' configuration, the length of coax between them, whether a teed stub should be used (notch filter), HAAT, and some other esoteric palaver. Two beams were selected, the cable cut, and up to the mountain it went.

Even I was surprised that is worked as well as it did. Via simplex, I could easily work into the tiny little mountain town to check on the skiing weather. Ah, but the surprise came a few weeks later, when the local FCC EIC figured out why the VHF police and fire frequencies were being interfered with. One would believe that beams, coax, and stubs all cut for 2 meters, would be enough to attenuate signals in the low 150 MHz range. In this case, not!

If you're thinking about a passive repeater, you have to jump through a lot of hoops, or....
Logged

KI4QPU
Member

Posts: 59




Ignore
« Reply #13 on: January 25, 2010, 11:19:03 AM »

I'm not an elmer, but do have some experience in the subject. I had a repeater that I could not hit due to a large mountain range between me and it. I bought a jetstream jtb 1 and it fixed my problem and then some. May look at getting one if a beam is not the avenue you want to go with.
Logged
K2DWB
Member

Posts: 15




Ignore
« Reply #14 on: January 31, 2010, 04:13:32 PM »

Thanks to all for the additional responses.  Out of curiosity, I got some extra masts today and elevated my J-pole an additional 5 feet and then an additional 10 feet...and did some comparison testing.

Strangely, the 5 feet boost proved superior to the 10 feet boost when working some distant repeaters (60 air miles +).  Nothing else was changed, such as antenna orientation, cabling, etc.  I'm perplexed.  Are J-poles so very position-sensitive that they might sometimes perform better at a lower height (again, excluding any cable change)?  There are no power lines, nearby antennas, tree branches, etc that would have interacted with my J-pole.  

I'm going to order a 5-element directional antenna tonight....quite curious to see how that works out.  Based on the responses here - and from some area hams who use directional antennas, I imagine I'll be pleased.
Logged
Pages: [1] 2 Next   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!