2m 1/4 wave - OK on 70cms as well? - UK Vintage Radio Repair and Restoration Discussion Forum

SiriusHardware · 6th Aug 2022, 10:05 pm

Just to add to the flurry of aerial questions circulating just now.

Today, I used my first ever VHF amateur band mobile aerial, a stainless steel 1/4 wave whip. When I was originally using it, it was with a converted Dymar 2000, a synthesised 2m-only radio. Great radio actually, very sensitive. I gave it to an elderly local amateur who had just passed his RAE and was using a donated set with just one channel crystalled for the local repeater.

I have several exotic purpose made dual-band mobile aerials made in Japan by the likes of Comet, but the simple 1/4 wave has the merit of being tough and relatively low profile, it could easily be mistaken for a PMR / Taxi radio aerial and if someone does lift it, another half-metre length of stainless steel rod won't break the bank.

My current radio is a dual band 2m / 70cms radio and I have always had in my head the idea that a 2m 1/4 wave is a 70cms 3/4 wave and that it may even have an acceptable impedance, ~50 ohms, on 70cms as well. Can anyone confirm this?

I understand that the radiation pattern of a 3/4 wave ground plane aerial is not ideal unless I plan to talk to the ISS on 70cms, but my main concern is whether I can safely transmit a 70cms signal with it if there is a need.

Jon_G4MDC · 7th Aug 2022, 9:38 am

It will match but it will have a high radiation angle.

There is a commercial dual band antenna which puts about 3/4 of a turn at the bottom making it a 70cm 5/8s and slightly shortened 1/4 wave on 2m.

The coil at the bottom of a 5/8 corrects the high radiation angle. From what you wrote I think you know this already.

SiriusHardware · 7th Aug 2022, 9:58 am

Thanks for confirming the bit about the high radiation angle, but I was less sure about the natural / native impedance of a 3/4 wave groundplane.

Dave757 · 7th Aug 2022, 6:06 pm

Hi,

I use an indoor Halo antenna for 2m horizontal working, and this
also works with a low VSWR for 70cm. I wouldn't like to speculate
regarding efficiency, but I still manage to get across to the continent
with 15W if conditions are reasonable.

Kind regards
Dave
G0ELJ

G6Tanuki · 7th Aug 2022, 6:19 pm

A high radiation-angle isn't necessarily a bad thing; I never did my commercial mobile-radio-stuff on 400-470MHz but one of my competitors did, and we happily interchanged information, on the basis that we weren't competing in the market so what was there to lose.

In an urban environment - let's say inner-city London, or maybe Reading, high-angle radiation was a good thing - it got to bounce-off high-rise office-blocks and tower-blocks of flats, so got you a RF path out.

Pragmatically, there's no point optimising your antennas to get max-radiation at 5-degrees-to-the-horizon if that takes you straight into a concrete-and-steel-framed shopping-mall.

SiriusHardware · 7th Aug 2022, 6:24 pm

I do most of my mobile operation static-mobile in high-up, open locations where a high radiation angle would put most of my signal into space, but it is just useful to know that if the 1/4 wave rod is the only antenna I have on me I can transmit into it without doing the transmitter any harm.

Sparky67 · 7th Aug 2022, 10:10 pm

Your question got me wondering as well SH... So I ran a plot for a 2m quarter-wave rod with a flat ground-plane (to simulate a car roof) using the MMANA software, but with four radials rather than a continuous sheet. Results are attached. Must admit I was a bit surprised how well the VSWR held up on 70cms according to the software. Would be interested to see a comparison with real life.

Martin
G4NCE

SiriusHardware · 7th Aug 2022, 11:00 pm

I don't have access to any of that kind of software, I completely missed the boat with all the circuit / aerial simulation stuff when it originally came over the horizon. I'm always impressed by how proficient so many of you are with that kind of thing but I am probably way too old now to be able to get familiar with any of it.

Anyway: That signal plot does look noticeably more buxom on 70cms, as had been suspected.

6th Aug 2022, 10:05 pm	#1
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,585	2m 1/4 wave - OK on 70cms as well? Just to add to the flurry of aerial questions circulating just now. Today, I used my first ever VHF amateur band mobile aerial, a stainless steel 1/4 wave whip. When I was originally using it, it was with a converted Dymar 2000, a synthesised 2m-only radio. Great radio actually, very sensitive. I gave it to an elderly local amateur who had just passed his RAE and was using a donated set with just one channel crystalled for the local repeater. I have several exotic purpose made dual-band mobile aerials made in Japan by the likes of Comet, but the simple 1/4 wave has the merit of being tough and relatively low profile, it could easily be mistaken for a PMR / Taxi radio aerial and if someone does lift it, another half-metre length of stainless steel rod won't break the bank. My current radio is a dual band 2m / 70cms radio and I have always had in my head the idea that a 2m 1/4 wave is a 70cms 3/4 wave and that it may even have an acceptable impedance, ~50 ohms, on 70cms as well. Can anyone confirm this? I understand that the radiation pattern of a 3/4 wave ground plane aerial is not ideal unless I plan to talk to the ISS on 70cms, but my main concern is whether I can safely transmit a 70cms signal with it if there is a need.

7th Aug 2022, 9:38 am	#2
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: 2m 1/4 wave - OK on 70cms as well? It will match but it will have a high radiation angle. There is a commercial dual band antenna which puts about 3/4 of a turn at the bottom making it a 70cm 5/8s and slightly shortened 1/4 wave on 2m. The coil at the bottom of a 5/8 corrects the high radiation angle. From what you wrote I think you know this already.

7th Aug 2022, 9:58 am	#3
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,585	Re: 2m 1/4 wave - OK on 70cms as well? Thanks for confirming the bit about the high radiation angle, but I was less sure about the natural / native impedance of a 3/4 wave groundplane.

7th Aug 2022, 6:06 pm	#4
Dave757 Heptode Join Date: Apr 2009 Location: Scratby, Norfolk, UK. Posts: 651	Re: 2m 1/4 wave - OK on 70cms as well? Hi, I use an indoor Halo antenna for 2m horizontal working, and this also works with a low VSWR for 70cm. I wouldn't like to speculate regarding efficiency, but I still manage to get across to the continent with 15W if conditions are reasonable. Kind regards Dave G0ELJ

7th Aug 2022, 6:19 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: 2m 1/4 wave - OK on 70cms as well? A high radiation-angle isn't necessarily a bad thing; I never did my commercial mobile-radio-stuff on 400-470MHz but one of my competitors did, and we happily interchanged information, on the basis that we weren't competing in the market so what was there to lose. In an urban environment - let's say inner-city London, or maybe Reading, high-angle radiation was a good thing - it got to bounce-off high-rise office-blocks and tower-blocks of flats, so got you a RF path out. Pragmatically, there's no point optimising your antennas to get max-radiation at 5-degrees-to-the-horizon if that takes you straight into a concrete-and-steel-framed shopping-mall. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

7th Aug 2022, 6:24 pm	#6
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,585	Re: 2m 1/4 wave - OK on 70cms as well? I do most of my mobile operation static-mobile in high-up, open locations where a high radiation angle would put most of my signal into space, but it is just useful to know that if the 1/4 wave rod is the only antenna I have on me I can transmit into it without doing the transmitter any harm.

7th Aug 2022, 10:10 pm	#7
Sparky67 Heptode Join Date: Feb 2008 Location: Great Barr, Sandwell, West Midlands, UK. Posts: 589	Re: 2m 1/4 wave - OK on 70cms as well? Your question got me wondering as well SH... So I ran a plot for a 2m quarter-wave rod with a flat ground-plane (to simulate a car roof) using the MMANA software, but with four radials rather than a continuous sheet. Results are attached. Must admit I was a bit surprised how well the VSWR held up on 70cms according to the software. Would be interested to see a comparison with real life. Martin G4NCE Attached Thumbnails

7th Aug 2022, 11:00 pm	#8
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,585	Re: 2m 1/4 wave - OK on 70cms as well? I don't have access to any of that kind of software, I completely missed the boat with all the circuit / aerial simulation stuff when it originally came over the horizon. I'm always impressed by how proficient so many of you are with that kind of thing but I am probably way too old now to be able to get familiar with any of it. Anyway: That signal plot does look noticeably more buxom on 70cms, as had been suspected.