When I first looked at this dual band mobile antenna design I had thought that on 2 meters the whole electrical length was in use and then on 70cm it was only seeing the top half but no measurement seems to work. For it to be a 2 meter 1/4 wave it would need to be around 50cm but once pulled straight it is much longer then the top section is longer than a half wave for 70cms , What is going on ?
Here is the design , I have just built a prototype for mobile and also built a base station design. Whilst 2 meters is the same performance as a 1/4 wave the performance greatly exceeds that of the 2 meter 1/4 wave on 70cms.
I think you are being very unfair to someone who is trying to learn. The measurements don’t line up with 1/4 wave on 2m or 5/8 wave on 70cm. And how is it matched to 50 ohm?
My answer is that the coil works as an inductor and that will electrically alter the lengths, but I don’t know beyond that.
True Richard, its great that Brad is building and experimenting with antennas but is this really SOTA? Or general antenna experimentation?
@G7SKU Its a 5/8 wave on 70cm. As you noted if you pull it straight its near 50cm. 50cm of wire is near 3/4 wave on 70cm, which will be give 50 ohms. Vast majority of 5/8 antennas are just matched by adding a base inductor to resonate as a shortened 3/4 radiator. Trick is getting the inductor dimensions & length to resonate on both bands, which DL5DBM has done in this design. Its a very common arrangement, there are loads of handheld 2/70 aftermarket antennas which are based on the same concept such as the Comet RH77 and many more around the 37cm to 40cm length. However you wont detect any benefit on 70cm vs using a standard 2m 1/4wave on 70cm. Fractions of a dB at best..
Thank you everyone , I hadn’t thought of the 5/8th wave with the matching coil at the bottom on 70cms now it’s been mentioned it is pretty obvious . Total length is about 55cm. I found the design as an image file and thought I’ll have a look on DL5DBM’s website but for some reason my PC will not allow me to access his site so I hadn’t a clue that there should have been another section to the drawing. I have never had the need to build a dual band antenna in the past. It does work really well compared to the 2 meter 1/4 wave used on 70cm as a mobile antenna and the 2 meter Dipole I was using in the loft ( not surprising) I am now hearing 70cm repeaters really well that just did not seem to exist in the past.In fairness the one in the loft has also had some descent coax added at the same time.
Here is my original test version which proved to be fantastic in performance on 70cm compared to the 2 meter 1/4 wave but a little impractical for mobile use ( I’ve ordered a couple of meters of stainless spring wire )
But if you wind the coil tight enough and small enough, you could use the antenna on an HT, or mounted on a pack frame.
The lengths don’t come out to nominal lengths because it isn’t the conductor length that matters in a coil, but the total inductance.
The problem with using a 2m dipole or quarter wave vertical on 70cm is that it radiates more power at higher elevation angles (generally useless for terrestial communication) due to the radiation from different parts of the radiator being out of phase and canceling to some degree at the horizon. This design reduces that because the portion of the radiator that contributes the most to the out-of-phase radiation is wound into a coil, which doesn’t radiate as much.
