w7lpn
07-03-2007, 08:11 PM
I hope Alan (http://k0bg.com/) doesn't mind
On Jul 1, 2007, at 7:33 PM, Rick F wrote:
Sir, I have been reading your site, and I really apreciate your knowledge & experience. My bug catcher seems to work very well, but I'm intrigued. Can I really get another 20 dB by improving my mount? Currently it is mounted on my tow stinger, with a UNUN 12.5 Ohms for 80-40 M & 28 ohms for 20M. This gets my SWR down ess than 1.5:1, I have a 1/4" ground braid from the stinger to the frame, the tail gate is in place, and I use one solid strand of #10 ga wire from the battery to the radio. My Bug catcher is 14.5 Ft with a 45 deg angle bent in the last 3 ft to keep the whip from hitting street lights. The lower mast is 4 ft, then then 80M coil, then a 1 ft mast stub above the coil, then a 2 ft capacity hat, then the 8.5ft stainless whip. I believe I need a wide ground strap, and maybe grounding my exhaust system between every joint. What other advice would you give. Tried to keep it short....
W7LPN
From: Alan Applegate <k0bg@plateautel.net>
To: Rick F <w7lpn@hotmail.com>
Subject: Re: Mobile Texas bug catcher
Date: Mon, 2 Jul 2007 06:30:10 -0600
I think you've done just about as good as you're going to get. After all, height is one major consideration, and there is just so high you can go while under way. Here's a little background that hopefully will get you in the right direction.
An HF mobile antenna will represent about 20 uF to 45 uF depending on its length (height as it were), and the frequency of operation. We cancel out this reactance by using a loading coil. Any capacitance we place between the antenna and the surfaces under it, and/or around it effectively shunts some of the RF to ground. We can compensate for some of the capacitive reactance by
adding more inductance in the coil. The net result is (it is much deeper than this) is the input impedance rises. No matter how you look at this, the rise in input impedance is loss that is turned into heat, not a radiated signal.
A ground mounted vertical does the same thing, but in most cases it is much closer to its image plane than a mobile one is. This means the amount of loss will be somewhat less. One way to decrease the coupling is to mount a vertical in the air, and string sloping resonant radials away from the antenna. Moving the antenna higher up on a vehicle has basically the same effect.
If you take any given antenna, and don't change anything about it except its mounting position (low, medium, and high say), the input impedance goes down, and so does the resonant point. Putting this a little closer to home, it is not uncommon to see the input impedance of an average 80 meter antenna drop from about 40 or 45 ohms mounted on a trailer hitch, to about 20 to 25 ohms mounted on the roof. The resonant point drops perhaps 10% to 15%. Again, this is because stray capacitance dropsIt should be apparent we have to make a compromise in a situation where we are already compromised. If you take nothing else from
this, the two main factors are length and height, and it is best to go as far as you can with both. You also might want to look at DX Engineering's Cap Hat with the four foot long rods.
As for the bonding; you need to do the exhaust at least three places. Don't forget to do the bed on all four corners. And as long as you have an RF return for the coax, further strapping to the mount will not help. By the way, the average mount has about 2 uF of capacitance, and there isn't much you can do about it
save for making the insulators thicker and larger in diameter.
You didn't state whether you had a high level of ignition RFI. You might want to read my article about that.
Alan
On Jul 2, 2007, at 11:44 PM, Rick F wrote:
Thanks so much for your time and Elmer-ing. I don't think I have ignition RFI, because noise is minimal while driving out in the country away from power-lines and there's not a notable noise drop when I kill the engine. It must perform fairly well because I
always get threw the noise when Net Control is complaining about conditions and I often relay for them for other weak base or mobile stations. Should I be pleased, or is this too unmeasureable? I hear many weak mobile signals and Net Control always tells me I have a nice signal. I will ground exhaust and bed as suggested. Thanks again.
P.S. I have heard claims that anything above the coil is wasted and serves only as inductive length. If my coil is large enough I should use a shorter whip & longer Mast? I've been told that I should have most of my length below the coil & cap hat, because the only radiating portion is below the coil. The whip supposedly does not radiate & cap hat minimally...?
W7LPN
The current at the base is high, and the voltage low and just the opposite at the top. Think about a right triangle with the antenna being the vertical leg, and the current being the hypotenuse. When you put a loading coil in the center of the antenna, the current in the base section still tapers off, but at a shallower angle. The current in the coil is uniform from end to end. The current flow in the top section (whip as it were) is again a right angle. This leads some folks to assume everything above the coil is wasted, and this is not true. The optimum position of the coil is based on a lot of factors. Some of them are, ground loss, length to diameter ratio, coil Q, and over- all length. If there were no ground loss, and the coil had infinite Q, it wouldn't matter where the coil was located. But there are losses, and we have to deal with them. Here is a program you might want to play with.
Mobile.exe (http://www.cplus.org/rmw/download.html)
with updates
Alan
Enjoy! http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif
On Jul 1, 2007, at 7:33 PM, Rick F wrote:
Sir, I have been reading your site, and I really apreciate your knowledge & experience. My bug catcher seems to work very well, but I'm intrigued. Can I really get another 20 dB by improving my mount? Currently it is mounted on my tow stinger, with a UNUN 12.5 Ohms for 80-40 M & 28 ohms for 20M. This gets my SWR down ess than 1.5:1, I have a 1/4" ground braid from the stinger to the frame, the tail gate is in place, and I use one solid strand of #10 ga wire from the battery to the radio. My Bug catcher is 14.5 Ft with a 45 deg angle bent in the last 3 ft to keep the whip from hitting street lights. The lower mast is 4 ft, then then 80M coil, then a 1 ft mast stub above the coil, then a 2 ft capacity hat, then the 8.5ft stainless whip. I believe I need a wide ground strap, and maybe grounding my exhaust system between every joint. What other advice would you give. Tried to keep it short....
W7LPN
From: Alan Applegate <k0bg@plateautel.net>
To: Rick F <w7lpn@hotmail.com>
Subject: Re: Mobile Texas bug catcher
Date: Mon, 2 Jul 2007 06:30:10 -0600
I think you've done just about as good as you're going to get. After all, height is one major consideration, and there is just so high you can go while under way. Here's a little background that hopefully will get you in the right direction.
An HF mobile antenna will represent about 20 uF to 45 uF depending on its length (height as it were), and the frequency of operation. We cancel out this reactance by using a loading coil. Any capacitance we place between the antenna and the surfaces under it, and/or around it effectively shunts some of the RF to ground. We can compensate for some of the capacitive reactance by
adding more inductance in the coil. The net result is (it is much deeper than this) is the input impedance rises. No matter how you look at this, the rise in input impedance is loss that is turned into heat, not a radiated signal.
A ground mounted vertical does the same thing, but in most cases it is much closer to its image plane than a mobile one is. This means the amount of loss will be somewhat less. One way to decrease the coupling is to mount a vertical in the air, and string sloping resonant radials away from the antenna. Moving the antenna higher up on a vehicle has basically the same effect.
If you take any given antenna, and don't change anything about it except its mounting position (low, medium, and high say), the input impedance goes down, and so does the resonant point. Putting this a little closer to home, it is not uncommon to see the input impedance of an average 80 meter antenna drop from about 40 or 45 ohms mounted on a trailer hitch, to about 20 to 25 ohms mounted on the roof. The resonant point drops perhaps 10% to 15%. Again, this is because stray capacitance dropsIt should be apparent we have to make a compromise in a situation where we are already compromised. If you take nothing else from
this, the two main factors are length and height, and it is best to go as far as you can with both. You also might want to look at DX Engineering's Cap Hat with the four foot long rods.
As for the bonding; you need to do the exhaust at least three places. Don't forget to do the bed on all four corners. And as long as you have an RF return for the coax, further strapping to the mount will not help. By the way, the average mount has about 2 uF of capacitance, and there isn't much you can do about it
save for making the insulators thicker and larger in diameter.
You didn't state whether you had a high level of ignition RFI. You might want to read my article about that.
Alan
On Jul 2, 2007, at 11:44 PM, Rick F wrote:
Thanks so much for your time and Elmer-ing. I don't think I have ignition RFI, because noise is minimal while driving out in the country away from power-lines and there's not a notable noise drop when I kill the engine. It must perform fairly well because I
always get threw the noise when Net Control is complaining about conditions and I often relay for them for other weak base or mobile stations. Should I be pleased, or is this too unmeasureable? I hear many weak mobile signals and Net Control always tells me I have a nice signal. I will ground exhaust and bed as suggested. Thanks again.
P.S. I have heard claims that anything above the coil is wasted and serves only as inductive length. If my coil is large enough I should use a shorter whip & longer Mast? I've been told that I should have most of my length below the coil & cap hat, because the only radiating portion is below the coil. The whip supposedly does not radiate & cap hat minimally...?
W7LPN
The current at the base is high, and the voltage low and just the opposite at the top. Think about a right triangle with the antenna being the vertical leg, and the current being the hypotenuse. When you put a loading coil in the center of the antenna, the current in the base section still tapers off, but at a shallower angle. The current in the coil is uniform from end to end. The current flow in the top section (whip as it were) is again a right angle. This leads some folks to assume everything above the coil is wasted, and this is not true. The optimum position of the coil is based on a lot of factors. Some of them are, ground loss, length to diameter ratio, coil Q, and over- all length. If there were no ground loss, and the coil had infinite Q, it wouldn't matter where the coil was located. But there are losses, and we have to deal with them. Here is a program you might want to play with.
Mobile.exe (http://www.cplus.org/rmw/download.html)
with updates
Alan
Enjoy! http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif