End Fed Antenna

[piratefm]

Hi Neckers

I'm after an antenna that will work between 88-108mhz . I'd like it to be an end-fed antenna with no tuning required so I can move around on freqs.

Do these exist? I'm sure Amphenol Procom did these a while back but can't be sure.

I'm not in a position to make one myself so ideally an off the shelf version to buy would be ideal.

Cheers

[radionortheast]

Think theres some end fed ground planes on ebay siros that are meant to wide band how well they work I don't know. I've got a wideband aerial I made myself in the loft, its a long rod next to a shorter rod, it definitely works better than trying to use a tuned antenna off frequency, kind of have a choke at the bottom and a wire that connects each element even thought its the loft it could get static build when outside so its done to protect the transmitter, its alright for a few watts if you can't find a frequency or want try different ones, I only use it with few watts at home, I think you do get a bit more downward radiation with such low power its not a problem.

[Albert H]

It's not really feasible to make a fully broadband aerial that would have any kind of efficiency - just consider the difference in wavelength between 97.5 MHz and 108 MHz - that's 3.43 to 2.77 metres - a difference of ~65 cm. You can make the radiating elements "fatter" to broaden the bandwidth (like using a "folded dipole"), but the increase in the range of frequencies over which the SWR remains usefully low is pretty small.

Just consider the amateur 2m band - 144 to 146 MHz. To get efficiency, most amateurs critically tune their aerials using a matching circuit so that their transmitter "sees" 50Ω, but the antenna varies somewhat over the band - and that's only over a 2 MHz spread.

Band II is a range of just over 20 MHz (a range of ~20%), so any truly "broadband" aerial is just going to be a radiating dummy load!

Probably the simplest aerial that's easily tunable is a quarterwave groundplane. You'll find that the SWR stays below 1.5 : 1 over a couple of MHz, but if you want to move it further than that, you're going to have to adjust the length of the radiating element (at the very least).

If you know what you're doing, you can make a frequency variable "Slim-Jim". I built a few of these out of threaded "studding", and used nuts and shake-proof washers to be able to move shorting straps between the vertical sections, and thus effectively vary the electrical length of the aerial. This worked OK, but the materials were relatively expensive, and re-tuning each aerial was a real pain.

In the end, I built a selection of J-poles, each tuned about 1.5 MHz above the last. I omitted a few frequencies that had high power "regional" stations on them, but I still ended up with a pile of fifteen aerials in my garage! These aerials were sold off over the years - each one trimmed exactly to the station's frequency. They worked very well, and I still see some of them about, even after nearly 30 years!

It's probably best that you choose two or three frequencies for your vicinity, and make two or three aerials accurately tuned to those frequencies.

[piratefm]

Thanks for your reply’s and very helpful to know Albert!

Wish I could make one myself, but can’t at the moment…annoyingly.

I have seen this one, the only one that I can find…unless someone else has any pointers for an End Fed?

Thoughts on this?
http://www.radiostructures.com/antennas ... d_238.html

[piratefm]

Thanks for your reply’s and very helpful to know Albert!

Wish I could make one myself, but can’t at the moment…annoyingly.

I have seen this one, the only one that I can find…unless someone else has any pointers for an End Fed?

Thoughts on this?
http://www.radiostructures.com/antennas ... d_238.html

Should add, I will get it cut to a frequency. I’m going to abandoned the wideband idea!

[Albert H]

That's a lot of money for a sleeve dipole! You can make one yourself (with a much better SWR than that commercial effort) out of a length of coax and a bit of wire....

Take a look at the "Flowerpot" aerials: https://vk2zoi.com/articles/half-wave-flower-pot/ These cost next to nothing to make, and can be built in various configurations - some of them with substantial gain.

It's worth doing some research on the radiation patterns of various aerial configurations. You'll find that (in simple terms) "the more metal you get into the air, the more radiation you get". I've frequently used ⅝ and ⅞ wavelength aerials, and a 0.63 X wavelength aerial has interesting properties too. The shaped coverage you can get from various aerials can really help with improving coverage from limited transmitter power.

The "Flowerpots" have a number of advantages, and can be constructed so that they don't look conspicuous in use. I have one on one of my houses that's on 98.4 MHz (effectively the middle of the band) which is used as a flagpole! It has an SWR of <1.05 : 1 from 97.9 MHz to 98.7 MHz, and is good for reception too.
I've made them using (for example) https://www.diy.com/departments/floplas ... 189_BQ.prd - 32mm PVC pipe.

You're going to need some very basic test gear to build aerials - at the very least you'll need a 1 Watt exciter on your frequency of choice and an SWR bridge. If you get a bit more serious about making aerials, it's well worth investing an "NanoVNA", and I use a home-made "Antenna Noise Bridge", which is really convenient. One noise bridge circuit is at http://qrpguys.com/k7qo-noise-bridge and another is at https://users.monash.edu.au/~ralphk/noise-bridge.html. Also Practical Wireless had a really good one back in the late 80s or early 90s.

[radionortheast]

an end fed dipole can be made fairly easily, thought it doesn't have any bandwidth at all just a sharp drop in swr on the frequency.

[yellowbeard]

January 1982 is the date for that practical wireless noise bridge Albert spoke of - it's available on this page:
https://worldradiohistory.com/Practical ... gazine.htm
You'd get reasonable efficiency and SWR for about 3-4 MHz either side of resonance using a slim jim antenna made from 1/2 inch copper tube, and it is kinda end fed...

[Albert H]

There was another Antenna Noise Bridge in PW that had a variable capacitor to allow determination of inductive or capacitive as well as the potentiometer to measure the resistive part of the impedance....

I'll see if I can find it.