[Antennas] Iron horse mobile antennas update

2002年6月03日 07:57:55 -0400

One additional comment. A three to one [3:1] VSWR is not uncommon for
short antennas. I use a LM-1 shunt inductor to achieve low VSWR on 40.
Dave, W1MCE
Dave Shrader wrote:
>> I'm using an IRONHORSE on 40 meter with the 706 MKII G. It tunes almost
> exactly as published.
>> My car is a Saturn SL-1 which uses a lot of plastic for fenders, doors,
> etc. So, my antenna is installed on the trunk lid away from all
> surrounding metal. The trunk lid is metal and I have used three bonding
> straps to get good RF ground.
>> The bandwidth at 2 to 1 VSWR is about 65 to 70 KHz.
>> I suspect that your installation is introducing too much capacitance
> from the vehicle body to the antenna. As I read your post the antenna
> seems to be from the trailer hitch up the side of the van. This means
> three to four feet or more of close mounting to the sides of the van.
> This is a 'high' capacitance connection. [Meaning you need a smaller
> section of the whip to achieve resonance.]
>> When you are adjusting the length of the whip you are changing the
> capaictance to the van body as the primary parameter with the self
> inductance changing as the secondary parameter.
>> You seem to be committed to the trailer hitch mount, so keep shortening
> the whip, from the base end, one inch at a time until the antenna tunes
> where you want it. [I did the same for my 20 meter hamstick]. You are
> correct in trying to keep the bottom of the the whip out of the coil.
> Hack off the base of the whip one inch at a time, with a hack saw, and
> test as installed on your van.
>> Note: The closeness of the antenna to the vehicle will introduce a
> varying VSWR during vehicle motion caused by the relative motion of the
> antenna to the van. [It swings back and forth on starting and stopping,
> it swings in the breeze as you drive due to wind load].
>> Dave, W1MCE
> _________
>> Ed wrote:
> >
> > Well I got the 706 in the van and a trailer hitch and mount installed. My buddy N4TOF and I put the 40 meter whip on and cannot get the swr down for anything. The hitch and mount are all grounded well. The mount is a good 8 inches away from the van. The antenna on 40 seems to only resonate with the whip way down in the coil which you can't have and it resonated only very high out of the 40 meter band. We can't get it to resonate anywhere with the whip in the recommended position. We did try trimming about 2 inches in all but don't seem to be getting anywhere. Is the antenna too long? I know if it resonates high it's supposed to be short but pulling it up in the shaft only results in a 3 to 1 in a very small bandwidth.
> > ED W4TEY
> > 325 CUMBERLAND DRIVE
> > HARROGATE, TN. 37752-6942
> >
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