Making a 300/450ohm Roll-up Slim Jim Antenna

[Polecat]

This is a great mobile/stealth antenna. And it costs £5-£6 to make, plus the coax.

I made both versions and thought I'd share my experience.

Range:
I got a mile out of the 300ohm version from 500mw in a heavily built up area. And that's with it dangling from the window. Haven't range tested the 450ohm version yet due to covid restrictions.

Measurements:
Use this calculator (thanks @albert h): xxxx Unless you can measure the actual velocity factor of your ladder line, use a vf of 0.90. Don't expect the antenna to be perfectly tuned based on these measurements. It's a starting point and if you have to make adjustments in length, it should be shortening rather than lengthening.

Additional equipment:
If you don't have an antenna analyser, invest in a nanovna. They're cheap and plentiful on ebay and will help you properly tune your antenna. The bigger v2 is out now but it's more expensive and no more accurate below 300mhz (says a v2 design team rf engineer I've spoken to). Also make or buy a patch lead and sma converters using the same coax you're using for your antenna. It calibrates your machine more accurately for your installation.

300ohm vs 450ohm:
300 is narrower than 450 so if stealth is important, that could be a deciding factor. Personally, I'd go for 450 every time due to its solid wire core vs stranded in the 300ohm. Cutting the feed points is particularly fiddly on the 300ohm. Also, on the 450, you can slide the insulation up and down on the cores when you're moving the feed point. So no matter how many adjustments you make, you'll only have one gap in your insulation. This isn't possible on the stranded 300ohm.

Construction tips:
Don't solder or tape it up until you're 100% tuned! They're extremely sensitive to minor changes in length, feed point and location.

In summary, if you're looking for an easy project and mobile or stealth is your thing, one of these could be right up your street.

[Polecat]

Link to calculator:

https://m0ukd.com/calculators/slim-jim- ... alculator/

[Santiago]

I'm going to give this a go

[Polecat]

I'm going to give this a go

You just made all that typing worthwhile!

Let us know how you get on.

[Santiago]

cable is here, just need to free up some time to try it out

[Polecat]

Great! Look forward to the next instalment.

[Santiago]

ok question,
at the bottom of the calculator it talks about a choke/balun for this antenna, x-amount of turns on a x-wide former to stop the cable radiatiating. anyone got a starting starting point for around 80mhz?

[Polecat]

I don't think it's critical. I tried several combos just binding the turns with cable ties each time until I got the best outcome. It didn't make a huge difference tbh but I landed at 5 turns and 40mm.

[Electronically]

I don't think it's critical. I tried several combos just binding the turns with cable ties each time until I got the best outcome. It didn't make a huge difference tbh but I landed at 5 turns and 40mm.

Yup if you do 5 turns on 40mm former. When it's all wound space them out till your around 390mm that should do it.

Sent from my AMN-LX9 using Tapatalk

[Santiago]

thank you, I will try that

[Electronically]

thank you, I will try that

Once you've spaced them out make sure you've measured it from one end to the far end if it doesn't reach to around 390mm spread them out further till it reaches around that.

Sent from my AMN-LX9 using Tapatalk

[Electronically]

thank you, I will try that

Once you've spaced them out make sure you've measured it from one end to the far end if it doesn't reach to around 390mm spread them out further till it reaches around that.

Sent from my AMN-LX9 using Tapatalk

One other thing is to try this as it's shorter in length. 2 turns. length should be around 63.5mm in total length. That should do it nicely.

Sent from my AMN-LX9 using Tapatalk

[Polecat]

thank you, I will try that

Once you've spaced them out make sure you've measured it from one end to the far end if it doesn't reach to around 390mm spread them out further till it reaches around that.

Sent from my AMN-LX9 using Tapatalk

One other thing is to try this as it's shorter in length. 2 turns. length should be around 63.5mm in total length. That should do it nicely.

Sent from my AMN-LX9 using Tapatalk

@Electronically

Thanks for jumping in on this. I'm happy to see I landed at something like the right measurement. It took me a long time to get a good reading on my nanovna.

You seem to be using a formula to get the length of coax to wind. Any chance you could share it or explain it?

[Santiago]

ok so I made it, here are some pics, ran it through the meter and not bad for a first attempt cant wait to test it on a big hill see what distance is can get needs a few tweaks for swr Any tips please? is the choke too close?

[Santiago]

thank you, I will try that

Once you've spaced them out make sure you've measured it from one end to the far end if it doesn't reach to around 390mm spread them out further till it reaches around that.

Sent from my AMN-LX9 using Tapatalk

One other thing is to try this as it's shorter in length. 2 turns. length should be around 63.5mm in total length. That should do it nicely.

Sent from my AMN-LX9 using Tapatalk

63.5mm? 6.35cm? that small? that would he two tiny loops

will that give me better swr than the 5 turn at 40mm you reckon?

[3metrejim]

I find making an antenna out of aluminium tubing quite time consuming and expensive. Is it possible to construct a vertical co-linear out of commonly available 75ohm co-ax rather than 300 ohm ladder line? I've got a few ideas, like replacing the the elements with shorted co-ax sections and making the phasing section out of a 1/2 wave length of co-ax, the matching section at the bottom out of two pieces of co-ax and rolled up to save space. Not sure about a ground plane (I like them as shields from RF below the antenna), but was thinking some construction like an upside down umbrella where the radials 'fold-out' when you want to set up. Do-able, or just wishful thinking?

[Polecat]

ok so I made it, here are some pics, ran it through the meter and not bad for a first attempt cant wait to test it on a big hill see what distance is can get needs a few tweaks for swr Any tips please? is the choke too close?

Looking good. Three comments:

The choke should be fine where it is.

Try hanging it up rather than testing when it's on the ground.

Have you tried moving the feed point up or down a bit? Keep moving in the direction it gets lower until you hit a good reading.

Unless you've already done that, in which case it's time to start altering the dimensions of the antenna.

I did say it's time consuming and not to solder until everything is spot on lol. Crocodile clips are good for testing different feed points....

[Santiago]

thanks polecat I was awaiting your reply, I've been stuck for spare time but I'm going to try all these things and get back to you lot with my findings

@3metrejim. thanks but that sounds a tad complicated for my very basic set

[Santiago]

ok so I moved the choke down a bit and added a other turn, unsoldered the feed point and had a little play around, finally managed to get the SWR to 1:1

need to give it a proper test see how far we can go

[Santiago]

the strip of wire was 2 quid, plus short length of rg58 plus connectors prob cost me about a fiver at best to build this