Hello all,
I’ve used and built VHF tuned antennas in the past such as the J-pole ( in various configurations) and Slim Jim but never as a method of attenuating out of band signals so my question is what sort of attenuation of out of band signals could you expect by using one ? This is just for my interest rather than anything else !
Many thanks all
Brad
VHF J-poles and Slim Jims are well suited for SOTA activations due to being omnidirectional in the horizontal plane and to their simple design (so they tend to be low-cost to buy or easy to make). But that means they are not particularly good in rejecting strong out-of-band signals as would unidirectional types like a Yagi or a loop antenna.
I assume you are asking because you might be suffering from breakthrough or receiver de-sensitivity from strong out-of-band commercial transmitters in the area. If so, you would probably be better addressing this at the radio using an add-on 2m filter. I don’t use one myself [I guess I’ve been lucky] but the SOTAbeam one has been praised on this reflector.
You got my purpose exactly right and as a lot of the comms on a nearby hill ( Titterstone Clee) are commercial Aircraft stuff ( along with many others) I was thinking of the reduced Harmonic radiation from my TX as well. The only way I can operated on 2 meters from this hill is to duck down below the summit which means I lose the omni directional ability of the antenna anyway and end up facing into an lower populated area. As I was using a Boafeng the first time, last weekend I tried my late fathers Yaesu FT1500m but this proved even worse with the RX just being killed permanently all the time I was up there ( The Boafeng was on and off when a particular transmitter kicked in. 70cms is totally unaffected. Someone had mentioned using a tuned antenna on one of my previous posts.
A single element antenna probably won’t give more then 10 dB to 20 dB reduction of out of band signals. It depends how far out of band the tx is. Besides if the signal is only 60 dB over 9 direct you still have to deal with a 40 dB over 9 signal. Rxs designed to scan wide frequency ranges have zero attenuation to out of band signals.
One of the old fashioned non scanning HH or a SOTA Beams filter are suggested as a practical solution.
73
Ron
VK3AFW
Clee Hill has multiple airband AM transmitters at the radar site running 50W or so to vertical cage dipoles. Several frequencies around 134MHz are used by London Control and these are likely to be very busy. Realistically the only way to kill that much unwanted RF so close to your RX frequency is to use a good quality multipole filter.
Thanks for the advise everyone, much appreciated. I think for the time being at least I’ll stay away from either Clee if using 2 meters. At least until I can reprogram my PMR radio to simplex frequencies or pick up another radio.
73 Armin
Hello Brad,
One solution that might be worth a try, is a Moxon antenna.
http://www.antentop.org/w4rnl.001/mvhf.html
I made one using 3/8" copper pipe and it is small enough to carry and robust enough to avoid damage.
And for your purpose, the antenna has a strong null behind it, rejecting signals coming from that direction.
Worth a try?
Ian
VE6IXD
Hi Brad,
Presuming you are using a HT, this antenna “kit” from BOX73 (I have one) works very much as a narrow band pass filter as well as a directional antenna being a magnetic loop for two metres.
[Portabel-Magnetschleifenantenne für das 2-Meter-Band]
(Portabel-Magnetschleifenantenne für das 2-Meter-Band)
At €16, it’s not expensive either
73 Ed.
That item has been out of stock for quite a while.
