Hourglass antenna

FM antenna discussion
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Paul_Logan
Posts: 307
Joined: 24 Sep 2014 12:58
Location: County Fermanagh, Ireland

Hourglass antenna

Post by Paul_Logan » 24 Mar 2020 19:50

So I've been messing around with a double delta loop for FM.

Original design here: http://www.arrl.org/files/file/QST/This ... tanley.pdf

Worked pretty good today during the E opening allowing reception of several firsts! With less pick up from local stations the E skip could sneak through on a couple of difficult frequencies.

Antenna doesn't have very good bandwidth unfortunately. it shows a 2:1 swr across about 2 mhz. The further you get from that the deafer it is.
Should work pretty good on an amateur band though!

My idea was to build something out of wire that would cover a spot frequency or the very bottom of the band and see what I could pick up with it during the E season - kind of QRP listening. Also it might be a simple cheap antenna for someone wanting to try FM DX for the first time. It's horizontally polarised and has a 50 ohm connection point - so perfect for SDR.

Well, bandwidth aside, it does work pretty good.

I've used thin insulated wire - obviously much thicker wire might help bandwidth.

I used 57 cm by 275.8 -came out resonant around 91.9.

Fun to play around with. Anyone got any ideas on how it might be improved?
Paul Logan
Fermanagh, N. Ireland

FMDX: Elad FDM-S2 with 5 el Yagi by K6STI, 9 el yagi.
MW / SW: Perseus / Elad FDM-S2 with EWE / Wellbrook Loop / MFJ1026 Phaser.

daveB
Posts: 1487
Joined: 07 Oct 2014 16:49

Re: Hourglass antenna

Post by daveB » 04 Apr 2020 16:01

Hi Paul,
I've tried modelling this in 4nec2. I can't manage to match your dimensions, I end up with a resonant frequency closer to 100 MHz for 2 mm diameter wire (which is roughly 4mm electrical cable). But then I'm not modelling any form of support and ground conditions willl affect it. I did a scaled up version of the dimensions from the ARRL article from 144MHz to get around 97 MHz resonant frequency.

The performance figures show a gain of 5dBd at resonance. Shifting the resonant frequency to mid band then gives a gain of 4dB at 88 MHz, 5dB at 98 dropping to 3.5 dB at 108 MHz. At resonance the nulls are a whopping -30 dB. At 88 MHz it gives 12dB fr/side null and the same at 106 MHz.

Modelled over "real ground" with the bottom of the aerial at 12 ft there is a noticeable ground gain, the resonant nulls are not quite so good.

As to improving performance - I wonder if fitting a 4:1 balun immediately before a pre-amp would help to reduces mis-match losses due to the SWR. Instead of the SWR climbing to over 60 at the band edges you end up with a maximum of 20, and around 4 at the resonant point. 12t primary and 6t secondary on a pig-nose/binocular core using 32 swg will work.

A nice reception log by the way

David
Locatio: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi 20ft agl - 120 deg. FM5 - 80 deg, FM5 -180 deg. OIRT dipole and 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz

Paul_Logan
Posts: 307
Joined: 24 Sep 2014 12:58
Location: County Fermanagh, Ireland

Re: Hourglass antenna

Post by Paul_Logan » 04 Apr 2020 23:14

Thanks so much Dave for modelling it. I calculated for 92 mhz and it's showing lowest SWR just below 91.9 on my RigExpert AA230. Support is non conductive. I was very surprised how well it actually worked. A balun I must try. Preamp would go against my initial idea of just wire, but can you suggest one? Impressed how well gain holds up across the band in your numbers. At first I thought it was terribly deaf well away from resonance however after doing much more listening on it I'd say the numbers you supply look about right. It's really not that bad even at the top of the band.

It definitely knocks back vertical signals well which is very good for me here ("The vertical currents cancel each other while the horizontal currents add in phase") The side null number at resonance is really interesting! Going to look at that a bit closer.

Thanks about the reception log. Can't wait to get trying it out again.

This was on the loop, a 100 watt Basque station so, definitely works: https://twitter.com/i/status/1246175214054055937
Paul Logan
Fermanagh, N. Ireland

FMDX: Elad FDM-S2 with 5 el Yagi by K6STI, 9 el yagi.
MW / SW: Perseus / Elad FDM-S2 with EWE / Wellbrook Loop / MFJ1026 Phaser.

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