So I managed to sneak in a little bit more work on the mobile install the other day, and here are some of the things I did:
- cut 5/8 and 1/4 wave whips for 2m
- fabricate new mount for NMO mount system
- piece together a temporary mount system for ATAS-120A HF antenna
- make stereo jack switch face plate for radio audio-to-vehicle audio junction
I started by cutting the 5/8 whip to find the lowest SWR at both 144.000MHz and 148.000MHz. The idea is that SWR will generally be at its lowest somewhere in the middle of a band such as 2m and so you are trying to balance the SWR at the band edges. Put another way, you want the low and high portions of the frequency range you are using to have as close to the same SWR as possible.
When I first tested the whip, I noticed that the SWR was very small on the low portion of the band and fairly large on the high portion of the band. This told me (from antenna theory) that high frequency = short antenna, thus a high SWR at a high frequency suggests a poor match because the antenna was too long. I tuned the antenna by cutting 1/4″ sections from the bottom of the whip and testing SWR at 144 and 148 MHz each time. To test, I set the radio power down to 5W (minimum) and briefly transmitted (FM) “VE3BUX test” while watching the SWR. I managed to achieve a nice low SWR at the band edges and a barely measurable SWR in the middle of 2m.
After cutting my 5/8 whip and confirming that the 1/4 wave whip was okay, I decided to fabricate a mount for a position lower on the vehicle – at the upper hinge for the rear door. I chose this location because while up high on the vehicle is great for radiation pattern, etc, it was not okay for apartment living (more specifically, having to park underground where our roof-rack only clears the ceiling by mere inches. I learned quickly that the FJ would look like a bumper car at the fair unless either: a) I removed the antenna prior to parking underground or b) mounted the 1/4 whip lower (for while in the city).
The next thing on my to-do list was to install the 3.5mm audio jack face plate I had put together to link the radio’s audio output to the FJ’s axillary audio input which will allow me to hear the radio over the vehicle’s speakers. I bridged the left and right audio channels since the output from the FT-857D is mono anyways.
This is what the audio face-plate looks like in its position. I will be making a short jumper cable with 90-degree elbows to link the radio audio out to the auxiliary input. I plan to use a hard-line to do the linking so I’ll post more once I have started that project.
Lastly, I snapped a picture of the radio in its mounting location. I have learned that it is not as easy as I had hoped to remove the radio from its current location – it looks like a second version of the mounting platform will be necessary to achieve the ergonomics I had in mind. I will be sure to keep everyone updated on what changes I made to the mounting location to facilitate the installation and removal of the radio.