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Wide Band Antenna for 100MHz-6GHz Hand-held Spectrum Analyzer

pnielsen

Member
I have purchased a secondhand Anritsu hand-held spectrum analyzer that covers the range from 100MHz up to 6GHz. The intended use is to take background EMI readings over that entire frequency spectrum for a total site assessment.

What options would allow me to perform these tests using the minimum number of attachable antennas? I would be willing to trade a certain degree of accuracy for the widest possible bandwidth display, and less accessories to fit and carry around.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The classic wideband aerial is the discone - which gives an exceptional bandwidth of ten to one - so assuming you can get them for the frequencies you need?, then two would more than cover your range.

I presume any degree of accuracy isn't important?, nothing is going to approach any degree of flatness over such a wide range.
 

pnielsen

Member
3GHz RX seems to be the typical upper limit for affordable ready-made discones. They are somewhat more bulky than I would prefer for use in the field with a hand-held spectrum analyzer.

This is the most compact production discone I have found so far. I could accept the 300MHz lower limit as a trade-off for smaller size.


This one handles up to my preferred 6GHz but looks expensive. If I cannot find an alternative solution, I may have to self-build or modify an exisitng model.

 

unclejed613

Well-Known Member
Most Helpful Member
biconical and planar disc antennas are good wideband antennas. the planar disc antenna is the simplest to build. the disc diameter for a planar disc is 1/4 wavelength at the lowest frequency of interest, and the high frequency limit is determined by the width of the feedpoint gap. http://www.wa5vjb.com/references/PlanarDiskAntennas.pdf

biconical antennas are similar to a discone, except it's two cones. if you do a search for biconical antennas, you will likely find a lot of hourglass shaped antennas that are intended to be used doing EMI/EMC testing. the limitation of a biconical antenna is it's SWR bandwidth is around 5:1 instead of the 10:1 or more possible with a planar disc.. i'm currently keeping my eyes peeled for 20' or 24' steel pizza pans. the planar disc i want to build will be for 144Mhz to about 1.5Ghz. my intention is to use steel pizza pans (because i do know how to solder to mild steel).

for 100Mhz, the disc diameter needs to be 75cm (about 30 inches), and i doubt you will find any pizza pans that diameter. you could also use copper tubing to make a pair of rings of the proper size, and "fill in" the "disc surface" with chicken wire soldered to the copper tubing.
 

pnielsen

Member
Thank you for those practical suggestions. I have also noticed log periodic antennas may cover a wide frequency range, but they are directional. May I could assemble an array of the same type facing in multiple directions for 360 degree coverage.

There is a good range available on eBay at relatively low cost.

 

unclejed613

Well-Known Member
Most Helpful Member
May I could assemble an array of the same type facing in multiple directions for 360 degree coverage.
there are a couple of "gotchas" there... the first one is the complexity. a Log-Periodic is more complex than a Yagi antenna (in both construction and math). you would also have interaction between adjacent antennas. depending on the geometry of the array, you might end up with sharp nulls in the radiation pattern and their depth and profile will vary a lot with frequency because, if you think about it, you will (assuming some sort of "star" formation of the antennas) have the lowest frequency elements interacting with each other with separations of small fractions of a wavelength, and separations of high frequency elements of several wavelengths. you may also get strange quirks of SWR at different frequencies because of these interactions.

if you are doing site surveys, a simple omnidirectional antenna is sufficient, which is why most site survey antennas are simple biconicals, planar disc, or discones. sometimes if a particular interference source needs to be localized, you would use a small Yagi to pinpoint the noise source. you're not looking for an interference source 20-30 miles away, but the ones up close that will be the most problematic. i looked at Anritsu handheld analyzers, (didn't see any that only went as low as "only" 100Mhz, most of them are 9khz to 6 (or more)Ghz. these analyzers have a noise floor of -160db or lower (with the 9khz-6ghz handheld (MS2713E) having a noise floor of -162db). that's a low enough noise floor that you would probably get useful info from the analyzer even if you just stick a paper clip in the input jack.
 

camerart

Active Member
Hi P,
I've just made a simple 'field strength meter' with a simple looped wire, which 'sees' background signals, plus test transmitted signals ok.

It's based on an LT5534.

Camerart.
 

unclejed613

Well-Known Member
Most Helpful Member
What can I do to stop high reflected power in my station
do you mean high SWR? high SWR is a sign of a mismatched antenna, either it's cut to the wrong length, or it doesn't have the right feedpoint impedance for the transmitter and transmission line (actually problems with the transmission line will also show up as reflected power).

so, if you have a transmitter with 50 ohm output impedance, and 50 ohm coax cable, and a normal center fed dipole cut to the correct length (1/4 wavelength for each half) it should be close to being matched. "usable SWR" is usually considered to be 3:1 or anything less (1.1:1 or 1.2:1 usually being about the best you can get) for solid state transmitters (some will balk at 3:1, but are okay below 2.5:1). tube type transmitters are a bit more forgiving, and i remember seeing people writing in QST a long time ago, that 5:1 was good enough for most tube type transmitters.

so, problems that will cause high SWR:
1) 50 ohm transmitter and 50 ohm cable and 300 ohm antenna (like a folded dipole, which is cut to the correct length to be resonant, but has a 300 ohm feedpoint impedance, corrected by using a balun between transmission line and antenna to match the impedance

2) 50 ohm transmitter, 50 ohm cable, and wrong length antenna. can be fixed by cutting the antenna to the correct length, or by using an antenna tuner (not as good a solution, but many ham operators operate on multiple bands, so this is what most of them use).

3) damaged transmission line. look for bends or crimps in the cable that might short the outer shield to the center conductor, or breaks in the cable, dog chewed portions, etc... the fix is to replace the cable or at least the section that's damaged.

there are other things, pieces of metal too close or touching the antenna, rusted/corroded connections, assembly screws, etc...

also, keep in mind an antenna has to have two connections to the transmitter, hot and ground. a piece of coax connected to the transmitter and only having the center wire connected to a long piece of wire is not a good antenna. the shield has to either go to a grounding stake (if you are using a vertical monopole) or another piece of wire to counterbalance the element connected to the center wire.

there are formulas for figuring out element lengths for whatever frequency, and even on-line calculators if you are not good at math. i sometimes use a "guesstimate" using the following method:

frequency/wavelength are related inversely proportional and in 3:1 or 1:3 using meters and hundreds of Mhz so there's always a product of 300
so, 3 meters is the wavelength of 100Mhz, and 1 meter is the wavelength of 300Mhz
30 meters is the wavelength of 10Mhz, and 10 meters is 30Mhz
300 meters is 1Mhz and 100 meters is 3Mhz

also doing the same math for the 1,5's and 2's, etc can be figured out easily and extrapolated if you are ok with doing math in your head. then for single elements you divide the wavelength by 4, or divide by 2 for half-wavelengths. using this method i can get an antenna "in the ballpark" in my head.
 

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