Tag Archives: coax

20m band End-Fed Choked Coax Dipoles (T2LT)

WARNING: This post has been replaced with the following analysis and design:

Banana Antenna Design May 2017

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The Resonant Feedline Antenna is also known as:

• Sleeve Dipole (& Flowerpot Antenna)
• Resonant Feedline Dipole (J Taylor, W2OZH)
• Tuned Transmission Line Trap, T2LT (CB folks)

For more about common mode chokes, see this article:

Pictures of this experiment follow including the 10-25 MHz >8K choke follow.

75 ohm to 50 ohm transmission line matching coax stub

Coax Transmission line coax stubs are frequency dependent. Making a stub for one frequency means it WILL NOT work for another frequency. My example is for a 20m Resonant Feedline Dipole, sometimes called a Sleeve Dipole or Resonant Coax Dipole or Tuned Choked Coax Dipole.

NOTE: CBers tend to call this T2LT. I have no idea why they refer to this antenna by that name because it stands for Tuned Transmission Line Trap which means it should be a TTLT – but then it doesn’t have a Trap? I digress. CB for you.


So you have an approx 75 ohm impedance antenna and you want to get the best match you can. Take the wavelength of the frequency, multiply it by the velocity factor of your 75 ohm matching coax and multiply again by 0.0815.

For example.

14.225 MHz = 21.089 metres
21.089 * 0.66 (what ever your velocity factor is) = 13.19
Multiply 13.91 * 0.0815 = 1.134m

Therefore, your transmission line coaxial transformer will be 1.134m long which is apparently about 29 degrees around the 360 degree circle.

Data found here: PA0FRI page.

Finally, I discovered MANY pages on eHam and QRZ forums of people asking the same question but most answers are with people answering questions which were not asked – or giving advice how to fix the antenna, or live with it. Why Americans need to argue the toss when others just need answers beggers belief :)

Bidirectional switchable 40m wire yagi

I switched on the other evening and heard a very quiet DX caller on 7.142. It was YC0LOU from Indonesia and I could only pick up parts of his call. He called and called and had no takers. In fairness, he was extremely quiet but as the sun was gradually moving around the sky, he finally became audible and it was worth giving him a shout. 400w off my inverted V at 7m height got his attention but I needed a few blasts for him to get my call right. I put him on the cluster and he had a pile up.

Now, the point is, had I had more gain, I’d have not only heard him better, but he’d have heard me quicker too.

So I could add more height to my Inverted V but the difference between 7m and 10m isn’t actually that much at 5 degrees off the horizon – not even a db. Hardly worth writing home about.

40m-wire-yagi-drawingAnyway, this was the QSO that made me sit up and take stock of what I could do. I was seriously considering phased verticals for DX when I thought up the idea of having a switchable wire yagi. Either firing East or firing West.

Like me, you may already have an inverted V dipole up for 40m, all you need to is build another one about a quarterwave in front – or behind your existing dipole but out of a single wire. You don’t need to feed this with coax, it’s a parasitic element, like a 2 element yagi.

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How to use dual coax feeders as ladder line

palstar-at4kHaving recently taken delivery of a Palstar AT4K manual tuner, I was keen to get her into production to replace my CG5000 in the attic.

Problem: the route to the attic from the shack is complex but I have a number of spare coax runs going that way including a couple of RG58 cables that I installed about 10 years ago as backups. Actually I originally installed three RG58 lines but I’ve been using one of them to send 12V up the line to the ATU.

parallel-cox-to-ladder-lineAfter MUCH research, I finally used about 20 feet of parallel coax feeders, connecting ladder line to both ends. To clarify, I run about 12 feet of ladder line from the ATU to the parallel RG58 cables. I soldered the ladder line to the inner core of the RG58 coax and shorted the braid-to-braid. My 20 feet of RG58 runs to the attic, through walls, up ceilings etc and in reverse, I connected the ladder line to the RG58. Again, I shorted the braids of each line to each other with a solder blob. My ladder line then has another run to the feedpoint of a large 60m loop that runs through the attic and around the garden.

The results have been quite amazing. Comparing my 40m reference dipole to the the CG5000 (SG230 type) ATU feeding the 60m loop has always shown that the loop was about an S point lower than my reference dipole for most stations.

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Aircell 7 coax review – vs- RG213

Dsc_0113I have used Aircell 7 now for a number of years and have finally replaced all my patch leads in the shack with precise lengths of Aircell 7 coax from SSB Electronics.

For longer runs, I’m still using Westflex and Ecoflex 15 but for portable operation, Aircell 7 is nice and bendy, easy to col up – and has pretty low loss, particularly for HF and short runs (up to 20m for VHF).

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Using Petroleum Jelly for temporary antenna connectors

I’ve been trying to find a product to protect my temporary antenna connectors for either Field Days or my holidays near the sea. The problem is two-fold; firstly water ingress to the coax and the connectors from rain and secondly corrosion. The corrosion issue only seems to occur near salt water and spray and occurs within a couple of days.

petroleum-jellyI used to use self-amalgamating tape for Field Days but the effort in applying and removing it forced me to rethink. I started using high quality insulation tape instead – the stuff that has some nice stretch and doesn’t go brittle in the cold. This worked for many years. Unfortunately, the cheap stuff, from say Maplins might have the required insulation properties but has a brittle plastic feel and not very pliable – it’s certainly difficult to make waterproof between layers. Last time out for Wythall Radio Club SSB Field Day, torrential rain found its way through a crack between two layers of tape and the SWR went high.

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Transmission Lines

I have an article brewing in me which is all about matching a low dipole for 160m to regular 50 ohm transmitter. The dipole I had in mind will be about 15 meters off the deck. My impedance will be about 20 ohms, maybe 25 if I can get it a bit higher. Basically, this is unnaceptable to me since I like to match things nicely.

Chris (G0EYO) sent me this link http://www.qsl.net/w4sat/qtrwavtr.htm which tells me that I need to insert a quarter-wave piece of coax that has an impedance of 35 ohms. Well, I don’t know about you, I don’t often have 35 ohm coax lying around, however a pair of 75 ohm quarter wave stubs will reduce the impedance of 75 ohms to 37.5 ohms. Near enough? I think so.

Thanks Chris, I think I worked it out now.


Waterproof PL259 connectors for Ecoflex

Waterproof PL259 ConnectorsI’ve been lucky enough to purchase 100 meters of Ecoflex 15 recently and the solderless connectors are awesome.

Available for Aircell 7 (RG213 replacement) and Ecoflex 10 (Westflex replacement) and this giant 15mm diameter stuff, once fitted they are completely waterproof.

My experience with all these products started with me laying down a few runs for my narrowboat earlier this year because I wanted a low RF leakage product. More recently, I’ve rebuilt most of my spare patch leads with the remainder of the Aircel and Ecoflex left over from the boat build. All my RG213 and Westflex has been relegated to tutoring and testing.


Coax Loss Comparison

I can never find the comparison charts between RG213 and Henry Westlake’s Westflex W-103. At last, I’ll have this logged forever now. Per 100 meters:

RG213/URM67(Mil spec)
Westflex 103
 100 MHz                      7 dB 3.2 dB
 144 MHz                    8.5 dB 4.5 dB
 200 MHz                     10 dB 5.4 dB
 300 MHz                     13 dB 6.2 dB
 432 MHz                     15 dB 7.5 dB
 1000 MHz                   27 dB 13.0 dB
 2450 MHz          use Westflex 103


 3000 MHz          use Westflex 103

27.8 dB

 5000 MHz          use Westflex 103

34.1 dB