[Waverley ARS] Antenna for Magnetic Mount

[Eric van de Weyer]

Hi Henrik,

Attached is a diagram for building a simple, dual band whip for 2m & 70cm.
It works as 1 quarter wave on 2m and as a 5/8 wave on 70. this is what I
have on my car and I have built quite a few over the years. I have
constructed them both from stiff wire and using a, fiberglass covered with
copper braid, whip, with a break at the bottom and the coil added between
the bottom and the braid. They have all worked well.

The antenna you found sounds like it is a UHF collinear and is probably too
short for use on 70cm but may be able to be used as the basis for one of
these antennae. It probably consists of 1 1/2 wave then the coil and then
another 3/4 wave above the coil. These usually have a gain in the vicinity
of 5.5dB.

The one above has basically unity gain on 2m and about 3dB on 70cm 

73....Eric VK2VE.

-----Original Message-----
From: members-bounces at us.cactii.net [mailto:members-bounces at us.cactii.net]
On Behalf Of Henrik Stenstrom
Sent: Tuesday, 4 March 2008 10:44
To: members at vk2bv.org
Subject: [Waverley ARS] Antenna for Magnetic Mount

Dear All,

Having had the fortune to avail myself of a mag-mount at the club
auction, I'm keen to press this into service by cutting a suitable
antenna for it.

Using the known formula and centre frequencies of 147 and 440 MHz
respectively, I've come up with the table below.

It seems that I have two length options for servicing both bands;

- Approximately 243mm as a 3/8 wave 70cm - 1/8 wave 2m antenna
- Approximately 486mm as a 3/4 wave 70cm - 1/4 wave 2m antenna

I also have what may be a suitable antenna which I found in the gutter,
possibly fallen off a taxi.  It has the standard screw connection, made
of wire and is about yea long.  At about half way along, it has some 6-8
helically wound turns.  

What is this spiral? Matching section?  Is it necessary?

I'd appreciate your comments and suggestions!

Regards,
Henrik - VK2HHS    

		147	440		147		440
		Feet	Feet		mm		mm
1/8	117	0.796	0.266		242.6		81.0
1/4	234	1.592	0.532		485.2		162.1
3/8	351	2.388	0.798		727.8		243.1
1/2	468	3.184	1.064		970.4		324.2
5/8	585	3.980	1.330		1213.0	405.2
3/4	702	4.776	1.595		1455.6	486.3
1/1	936	6.367	2.127		1940.8	648.4
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