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ANTENNA SWR QUESTION
- Thread starter darticus
- Start date
1. Use a good external meter.
2. Believe what it says.
3. Act accordingly.
2. Believe what it says.
3. Act accordingly.
Yes, you should at the very least recheck it. Your counterpoise has changed and that can affect the tuning of the antenna.
It is possible that you may need to do nothing, but it never hurts to check.
Also, use a good quality external meter. The ones built into most radios are often not even close to being accurate. Getting such a meter does not cost much.
The DB
It is possible that you may need to do nothing, but it never hurts to check.
Also, use a good quality external meter. The ones built into most radios are often not even close to being accurate. Getting such a meter does not cost much.
The DB
I agree with the DB. I have taken antennas from one vehical to another and they didn't need to be retuned except 1. But like he said, recheck it anyway. You don't want to find that you were the exception.
Yes it does. With a magmount, how thick the metal is it is attached to has a large effect on the coupling to ground. Also the size of the vehicle it is on and how much of it is horizontally metal again has an effect.
When I installed a HF antenna on my car, I noticed a 300kHz difference in the resonant point between when I installed it and after bonding the trunk lid and hood with braid straps as per k0bg.com instructions.
DEfinitely re-scheck it.
What is not always understood is that the vertical antenna is just half of the antenna system. The other half is the vehicle body.
Both halves radiate.
You would not expect a dipole antenna to have the same characteristics if you cut (or lengthened) one side. Ditto with changing vehicles!
All the best!
What is not always understood is that the vertical antenna is just half of the antenna system. The other half is the vehicle body.
Both halves radiate.
You would not expect a dipole antenna to have the same characteristics if you cut (or lengthened) one side. Ditto with changing vehicles!
All the best!
I have a cheapie 10 year old Hustler magnetic that I use for rental cars nowadays (I quit driving my personal vehicle)...I set the SWR with it installed on my Wife's Forester and I check it on every rental car I get...it has NEVER changed, and obviously; never needed retuned.
But I do check it every time...
But I do check it every time...
True statement.
Not necessarily true.
There is a difference between a quarter wave ground plane and a half wave dipole. The ground plane allows the quarter wave vertical to think it has a bottom half. This does not mean it radiates. A properly constructed ground plane will be set up as such that any radiation will be exactly out of phase with the radiation on the far side of it, and thus any radiation would cancel. This is not completely possible on a car. That being said, an antenna can be placed where the strongest currents, which would be in the metal near the antenna, effectively cancel thus very little radiates.
I am not saying that something isn't radiated from the car body, however having it radiate as little as possible is the best option. That is one reason centering the antenna on the roof is considered the best possible mounting place.
The DB
When you move the antenna from one vehicle to another check the match and tune accordingly. The swr might change a little or maybe a bunch.....it costs nothing to put your swr meter on it and check.
Do both halfs of a dipole radiate? Yes, Do both 'halfs' of a vertical antenna radiate? Yes. All antennas have 'two halfs'. Both radiate. Only one 'half' of an antenna can't radiate anything, it always takes two 'poles' or current doesn't flow. No current moving means no radiation. Sometimes it's a real 'trick' to figure out where that 'other half' is, but it's always there...
- 'Doc
- 'Doc
You are correct. I did use a very poor choice of words there. Those words were:
However, I also continued with:
Even if, hypothetically speaking, it is perfectly canceled out by the portion of ground plane that is exactly 180 degrees out of phase with it, it is still technically radiating.
However, most of this radiation (from the ground plane), in a car, will not be vertically polarized like the antenna, thus as far as horizontal radiation goes will already be heavily attenuated for a local (non-skip) vertical receiving antenna. There is also the point that a significant portion of this radiation is radiating exactly out of phase with itself, further attenuating its affect on any antenna. Thus, while RF is still being radiated from the ground plane, its effects on a receiving station is tiny to negligible.
What I was responding to was something I thought that kt4ye implied. I took it as him referring to a quarter wave vertical and a half wave dipole being exactly the same.
The DB
And since RF is AC, when the other half of that cycle comes around, when that (+) signal from the vertical part of the antenna changes to (-), does the vehicle's body becoming (+) then radiate? Or does that (-) signal then radiate from the vertical part of the antenna? If that (-) does radiate from the vertical part of the antenna, why then wouldn't it radiate when the vehicle's body was (-)? Or, change that other half of the antenna from a vehicle's body to radials, or the opposite side of a dipole, they both radiate no matter the form that 'other half' takes.
- 'Doc
- 'Doc