Building a radio antenna

General Questions
#1

I want to make a whip antenna for my portable ham radio. I’ve calculated the optimal length as 19.25" so I was going to get a sma female connector and some RG58 coax. I know that for long runs I want something less lossy but for less than 2’ I figure RG58 is the best compromise between loss and durability of the whip. Would RG8 be signifactly better?

Anyways, I figure there are three options with prepping the coax as an antenna

  1. Strip the shielding completely
  2. Leave the shielding in place
  3. Pull the shielding but leave it attached as a pigtail

Would there be any difference between these three options and if so, which one is best and why?

#2

what antenna have you made, does it have a name?

if you leave the coax shielding in place it won’t be much of an antenna.

there are some designs that leave all or part of the shielding configured in a particular fashion.

#3

Yes, if you are trying antennae having more or less of the shield in place as an active component, there are all sorts of variants. Many tie the shield and center conductor together at certain spots. There have to be numerous articles online about this - maybe the ARRL is a good place to start.

#4

Are you making the antenna out of the coax or using the coax as a jumper between the radio and the antenna?

For an HT whip, the ground/shield isn’t going to come into play RF-wise and the outer part of the connector is for mechanical fastening reasons. Coax probably isn’t a good building material here.
On a conventional HT, the chassis of the radio itself acts as the ground plane which provides something to ‘transmit against’ as well as pull the pattern down a little. Of course, you’d be best off with a 19.25"+ copper disc at the base of the antenna (at 100 feet) but antenna design is the art of compromising.

I do have the ARRL Antenna Handbook mentioned by Napier and it is quite complete. It is one of those books that puts a lot of expensive ideas into your mind. “Oooh, there’s a chapter for satellite comms. And, hey, maybe I should get a moonbounce rig going…”
Let me know if anyone needs any particular information from it.

#5

He’s asking how to turn a 2’ stub of coax into an antenna.

You must remove the shield from that 2’ of coax, for it to work.

The coax type (RG whatever) is mostly in the shielding quality - that is not very relevant when you remove the shield , is it ? Well the cheaper stuff may have a better insulator material, mechanically better…

Yes you need to remove the shield wire from the coax, or else the shield is the antenna.

Leave the shield as a pigtail so that you can ground any base plate if you want, although of course its a simple ‘coat hanger’ job to connect the base plate to the coax style connector as needed.
What about the impedance mismatch issue ? the wire in the air is 300 ohm, the coax is 75. There should be a balun ?

#6

That would make it hard to hold up to my ear.

RG58 is 50 ohms

#7

The impedance of a quarter wave monopole above a ground plane, which isn’t exactly what the OP will be building but should reasonably close, has an impedance of 38.6 Oms. A half-wave dipole has an impedance roughly twice that.

It’s a portable Ham radio, and it’s hard to say if the radio itself would act like a ground plane or like the second wire of a dipole, but since a 50 Ohm line is right in the ballpark of both those numbers, it should work reasonably well.

#8

http://www.cqham.ru/rubberduck.htm

#9

That is exactly the sort of antenna I’m trying to replace.

So I don’t know if I have the answer yet. Shielding on, off or as a pigtail?

#10

If you are building a whip and insist on using coax (for some reason), the coax shield does not, again, do anything for transceiver performance.
Whip here means single, vertically oriented conductor. A monopole.
It would be different for a balanced antenna like a dipole or a jpole or colinear.

So no shield on a whip.

#11

for what you seem to want then leaving the shield on makes it a non-antenna. coax is intended to minimize signal loss.

#12

Well, the link gives you instructions on how to make a rubber duck, pretty economically. How cheap are you trying to be? If really really cheap, why not just through a random wire on the end? It may not have that much worse of an SWR than a crude rubber duck.

The conventional antenna design would have the shielding off, I think, because the whole point of shielding in the first place is to prevent losing power to radiation, i.e. it is to prevent the transmission line from turning into a (bad) antenna. But actually figuring out what would happen is a little tricky. Thing is, the end of this coax would be unterminated. So I’m a little unclear on what would happen to an RF signal across the conductors in the last 1/4 wavelength before the end. Seems like a good second-semester E&M exam question, actually.

#13

Why even bother with the coax at all?

From what is being described here, you are basically making a quarter wave whip for the 2 meter band. Just solder a 19.75" piece of solid wire to the center pin of your connector and be done with it.

For the record, collapsible whips for 2m are available commercially from just about any amateur radio store, and aren’t expensive.

Also for the record, you won’t get much improvement over the rubber-duck antenna with a plain 1/4w antenna… For noticeable RX/TX improvement, the standard add-on for an HT is a 5/8 wave whip with a small matching coil at the base. YMMV.

#14

To be honest, I’m new to ham and I want to play around with making antennas to learn how to do it. My first is the whip and after that a whip with a pigtail. After that I’m putting up a dipole and running it via coax to my handie-talkie Baofeng. Then the horizontal wire between 2 trees antenna.

What is this matching coil of which you speak? Do you mean a balun = 5/8 lambda?

#15

Oh and just to clarify, I double checked my calculations and it is 20.25" which is approximately a 1/4 wave antenna at 146Mhz and a 3/4 wave antenna at 435 Mhz.

#16

you can get a plan to make a specific antenna. get one for a 2m 1/4 wave whip.

you should get a book on antennas, buy one or borrow one. read it and get to understand antennas. you can then decide on projects to try.

you can ask questions on this and other forums. without some knowledge about antennas you won’t even know what good questions to ask. to build antennas or have some background to follow plans you need to understand some antenna theory. now people can spend one or three lifetimes doing antennas, people make careers about doing antennas, people spend every waking minute hobbying about antennas. you don’t have to go nuts. read some general theory, pick a limited specific area to learn more detail; if you have a 2m radio then 2m antennas are what you should do.

here’s a 2m ground plane. what you want to do is that without the radials, just a wire on the center pin (you can use coax with the jacket and shield removed). you also will be using a connector that fits your radio.

http://www.hamuniverse.com/2metergp.html

websites like the above will provide lots of additional information and plans. you may also find a ham radio club in your area or for 2m specifically a repeater club (a group that sponsors a 2m repeater, which is another aspect of ham radio). people in these groups by their nature and purpose are friendly and helpful.

you would want a horizontal antenna on 2m only if you were using AM/SSB. if you are using FM then stick to vertical antennas.

ham radio is a hobby that is unlimited for money, time and energy. learn a bit and then find what you want to start with. give yourself some limits to not feel overwhelmed.