why my rod antennas couples much low frequency?
The question comes from installing a rod antenna to homebrew wideband receiver(20Mhz-1Ghz).
I built this receiver with spectrum analysis ability. so, I could see anything in the band.
In initial phase of work, I use an used signal source from R&S to test the receiver. The spectrum seems to be clear and noise plateau is relatively flat. Today, I install a rod antenna, see pic1,
when i try to look the real world. The real world sounds to too roaring in VHF band. see pic2.
(line in blue is unload antenna"s spectrum, green line is spectrum after antenna installation.)
It's obvious that the antenna pick up an noise hump in vhf band. I was very curious why there are so much noise or spectrum product in vhf. I install the same antenna to a HP's spectrum analyzer. To compare that, the HP's result seems to be good, and the noise looks much better in vhf band.
I made some experiment on this antenna and found that the antenna's ground play a key role in this vhf noise hump. The result is that if I directly contact the rod antenna's ground to HP analyzer's ground (RF input connector), the hump will disappear without doubt.
see below pic.
However, if I directly contact my receiver"s shield box (ie., power ground) to HP analyzer's ground. The hump would reduce, but won't as effective as directly contact antenna's ground to spectrum analyzer. Humm... I must say, I don't know why, and I want to consult.
Tow things I must to mention are
1. rod antenna is connected to my receiver with SMA terminated cable, the cable seems good. I once directly plug antenna to receiver's antenna port. The hump persists there.
2. my receiver is powered by 18V+ AC/DC converter, the DC's negative seems to internally short to 220V AC's GND in converter itself. And my receiver shared the same AC power supply board with spectrum analyzer. So, I guess they shares the same ground.
I'm not sure why this happens, but my receiver seems to be noisy in face of rod antenna.
Any idea or any answer would be welcome. Thanks advance!
And I would continue to try after my dinner. And hope to know why.
Jeff
That maybe the noise floor of your SA, and in real world there are many signals
Compare with a short and long monopole antenna and you will find many digital TV channels in the upper VHF and UHF range are quite strong.
What you see is several effects overlapping.
1. Your receiver may have a flat frequency response if you tested it from a calibrated and MATCHED RF signal generator.
2. Connecting a rod antenna brings the real "life" to your receiver. A rod antenna is almost NEVER MATCHED which causes the rippled response.
3. The spectrum is full of signals one may not expect, and some are quite intense like FM and TV, also short-wave signals can enter as well as their harmonics.
4. A rod antenna is good to receive strong signals from all directions. Use a good directional antenna (good means designed to be matched at a desired frequency band). Point such antenna to a desired transmitter location. Using a rod antenna brings many unexpected signals to your receiver. An adjustable attenuator may be good to reject unwanted signals from entering the receiver.
5. To make a good antenna-receiver combination requires to plan the system purpose, to learn what signals come to your location, etc.
just a suggestion
rod normally refers to a ferrite rod antenna.
i take it you mean a telescopic antenna ,
rod antennas = ground plain antenna = 1/4 wave, 1/2 wave, 3/4 wave or 1/8, 1/16, 1/32 ect
for best results they are cut or pulled to the required frequency band or needs.
an maybe coil or capacitor used to tune them to frequency of use.
the input Z of a 1/4 antenna with ground plane is good because
it can be pulled to the right match 50R.
ground planes are required. this has to be about a 1/2 wave length
wide.3 or 4 1/4 radials or a metal sheet will do at high frequencies.
the received radiation direction is 360 deg horizontal. horizontal polarized
the wavelength is approx 300^8 /F to start.
its not a wide band antenna it has its favored frequencies
and it will reject some it don't go on that's why you see the ripple
in the spectrum. of course not only caused by the selectivity
of the antenna but also the background noise.