The AM Forum

I just bought a 102 inch stainless steel whip from Radio Shack (yes, they're still in business!). It was originally meant as a 1/4 wave whip for CB. I would like to make a 40 meter mobile antenna out of it centered around 7.293 MHz. How do I calculate the needed base loading coil for it?

The new minivan is gonna look so awesome!!!

Jon
KA1TDQ

J,

I grabbed my '68 ARRhllL handbook and gave it a quick look.  For a 8' whip it gives -  20 uH at 7200kHz.  34T of #12 on a 2 1/2 core over 4 1/2 inches. I also have a program app on my Windoze 98 machine. If you'd like, I can maybe get it alive, and run out some numbers for you. Wire size, matching coil(s) spacing, type of car stuff like that.

Then there is the dog log type of ant.

klc

Ha! The dog log... Since I'm not driving a cadaver-lac, I'll need to be a little more low-key. :-)

20uH is good enough figure is good enough for me. I still need to figure out what I'm going to use for coil material, diameter, and stuff. I'll make it like 21uH and find the correct tap position from there.

There's a mountain that is just a couple miles from my house that you can drive up. It has an elevation of 2690' (called South Mountain in Phoenix). Some mobile AM might be fun with the boy when he gets old enough.

I'll have to make this a mag mount with 4 scoteful magnets. The minivan has to fit in the garage, so the antenna has to come off at night.  The base coil and mounting structure will probably be a Home Depot special.

Jon

John,  propane fittings make fine business quick disconnects.   Dollar store cutting boards make fine antenna mount insulators.

Ask me how I know....   Lol


--Shane
KD6VXI

Well, ifin the XYL will let you, mount your ant on the upper side of the van.  I tried the mag mount mobile way. Worked FB for the 2m CB,  not bad for 6m and worked on 10m.  I spent over 15 years operating mo bile, with crappy antennas, and worked out ok. I even made a few AM contacts with a 40m hamstick type ant, but nothing to write home about. (this antenna is probably the worst type to use - the Gotham of mobile opps). I had it on a pick up, so there was some metal actually under the 'stick.

You really want to establish a 'ground' to the body of the vehickel.

The mag mount capacitly couples to the body, and works for higher frequencies. Your van is the other end of your antenna, so on 40m you'll need a lot of magnets. Putting the ant up and away from the ground couples less of the energy to the van body and the cold, lossy ground. Leave your mount, and pull the antenna off when its car beddie bye time.

This site has lots 'o info.

http://www.k0bg.com/

But do come back, and don't drink too deeply of the ssb and fm cool aid.


klc

Here's a picture of the desert-mobile. I'll have to use a mag mount antenna, but a 3 or 4 magnet type.

I just tore apart my single tube transmitter to re-use the welded steel chassis. I'll never be able to fabricate one as well again. And, since I'm not going to build using tubes anymore, it just makes sense.

I'm going to use that chassis to build a mobile rig. I'm aiming for 80 watts carrier. Basically, I'm going to copy my single FET CW rig and then modulate it.

Jon

Jon

South Mountain is a RF nightmare. The look out furthest away from the broadcast sites seems to be ok.
Mt. Ord on the sr-87 heading towards Payson is one of the best spots for mobile dxing. Very quiet and cooler in the summer.

73's

Sam

True, there's almost 30 towers with everything known to man RF-wise up there. Mount Ord like you say is probably good, and I bet Mount Lemmon in Tucson is ok. That's almost a mile high and you can drive up it.

Jon

Hey Jon,

What's up with those weird looking trees in the background.  Just exactly where is your QTH??.  We don't have any weird trees like that here on the east coast.;D

Anyways, OK FB on the XYL and second harmonic.  Good luck with your new mobile rig project and antenna.

Fred

 " weird looking trees in the background "

Must be the road salt stunting the growth.

klc

Mt. Lemmon is probably ok. Depends on how far you want to drive. There are also good places outside of Showlow and Pinetop.

...the once proud 7984 tube rig! Now waiting to be made into a FET masterpiece! Insert some cheesy French mumbo jumbo here followed by a smacking of the lips with fingers clenched together.

Jon